2138 lines
75 KiB
Markdown
2138 lines
75 KiB
Markdown
# Python {#python}
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## Reference {#reference}
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### Interpreters {#interpreters}
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@python-interpreter-table@
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The Nix expressions for the interpreters can be found in
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`pkgs/development/interpreters/python`.
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All packages depending on any Python interpreter get appended
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`out/{python.sitePackages}` to `$PYTHONPATH` if such directory
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exists.
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#### Missing `tkinter` module standard library {#missing-tkinter-module-standard-library}
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To reduce closure size the `Tkinter`/`tkinter` is available as a separate package, `pythonPackages.tkinter`.
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#### Attributes on interpreters packages {#attributes-on-interpreters-packages}
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Each interpreter has the following attributes:
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- `libPrefix`. Name of the folder in `${python}/lib/` for corresponding interpreter.
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- `interpreter`. Alias for `${python}/bin/${executable}`.
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- `buildEnv`. Function to build python interpreter environments with extra packages bundled together. See [](#python.buildenv-function) for usage and documentation.
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- `withPackages`. Simpler interface to `buildEnv`. See [](#python.withpackages-function) for usage and documentation.
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- `sitePackages`. Alias for `lib/${libPrefix}/site-packages`.
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- `executable`. Name of the interpreter executable, e.g. `python3.10`.
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- `pkgs`. Set of Python packages for that specific interpreter. The package set can be modified by overriding the interpreter and passing `packageOverrides`.
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### Building packages and applications {#building-packages-and-applications}
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Python libraries and applications that use tools to follow PEP 517 (e.g. `setuptools` or `hatchling`, etc.) or
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previous tools such as `distutils` are typically built with respectively the [`buildPythonPackage`](#buildpythonpackage-function) and
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[`buildPythonApplication`](#buildpythonapplication-function) functions. These two functions also support installing a `wheel`.
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All Python packages reside in `pkgs/top-level/python-packages.nix` and all
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applications elsewhere. In case a package is used as both a library and an
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application, then the package should be in `pkgs/top-level/python-packages.nix`
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since only those packages are made available for all interpreter versions. The
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preferred location for library expressions is in
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`pkgs/development/python-modules`. It is important that these packages are
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called from `pkgs/top-level/python-packages.nix` and not elsewhere, to guarantee
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the right version of the package is built.
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Based on the packages defined in `pkgs/top-level/python-packages.nix` an
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attribute set is created for each available Python interpreter. The available
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sets are
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* `pkgs.python27Packages`
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* `pkgs.python3Packages`
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* `pkgs.python39Packages`
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* `pkgs.python310Packages`
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* `pkgs.python311Packages`
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* `pkgs.python312Packages`
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* `pkgs.python313Packages`
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* `pkgs.python314Packages`
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* `pkgs.pypy27Packages`
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* `pkgs.pypy39Packages`
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* `pkgs.pypy310Packages`
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and the aliases
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* `pkgs.python2Packages` pointing to `pkgs.python27Packages`
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* `pkgs.python3Packages` pointing to `pkgs.python312Packages`
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* `pkgs.pythonPackages` pointing to `pkgs.python2Packages`
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* `pkgs.pypy2Packages` pointing to `pkgs.pypy27Packages`
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* `pkgs.pypy3Packages` pointing to `pkgs.pypy39Packages`
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* `pkgs.pypyPackages` pointing to `pkgs.pypy2Packages`
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#### `buildPythonPackage` function {#buildpythonpackage-function}
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The `buildPythonPackage` function has its name binding in
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`pkgs/development/interpreters/python/python-packages-base.nix` and is
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implemented in `pkgs/development/interpreters/python/mk-python-derivation.nix`
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using setup hooks.
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The following is an example:
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```nix
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{ lib
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, buildPythonPackage
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, fetchPypi
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# build-system
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, setuptools
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, setuptools-scm
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# dependencies
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, attrs
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, pluggy
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, py
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, setuptools
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, six
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# tests
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, hypothesis
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}:
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buildPythonPackage rec {
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pname = "pytest";
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version = "3.3.1";
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pyproject = true;
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src = fetchPypi {
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inherit pname version;
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hash = "sha256-z4Q23FnYaVNG/NOrKW3kZCXsqwDWQJbOvnn7Ueyy65M=";
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};
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postPatch = ''
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# don't test bash builtins
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rm testing/test_argcomplete.py
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'';
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build-system = [
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setuptools
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setuptools-scm
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];
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dependencies = [
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attrs
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py
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setuptools
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six
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pluggy
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];
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nativeCheckInputs = [
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hypothesis
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];
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meta = {
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changelog = "https://github.com/pytest-dev/pytest/releases/tag/${version}";
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description = "Framework for writing tests";
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homepage = "https://github.com/pytest-dev/pytest";
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license = lib.licenses.mit;
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maintainers = with lib.maintainers; [ domenkozar lovek323 madjar lsix ];
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};
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}
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```
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The `buildPythonPackage` mainly does four things:
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* In the [`buildPhase`](#build-phase), it calls `${python.pythonOnBuildForHost.interpreter} -m build --wheel` to
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build a wheel binary zipfile.
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* In the [`installPhase`](#ssec-install-phase), it installs the wheel file using `${python.pythonOnBuildForHost.interpreter} -m installer *.whl`.
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* In the [`postFixup`](#var-stdenv-postFixup) phase, the `wrapPythonPrograms` bash function is called to
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wrap all programs in the `$out/bin/*` directory to include `$PATH`
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environment variable and add dependent libraries to script's `sys.path`.
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* In the [`installCheck`](#ssec-installCheck-phase) phase, `${python.interpreter} -m pytest` is run.
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By default tests are run because [`doCheck = true`](#var-stdenv-doCheck). Test dependencies, like
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e.g. the test runner, should be added to [`nativeCheckInputs`](#var-stdenv-nativeCheckInputs).
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By default `meta.platforms` is set to the same value
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as the interpreter unless overridden otherwise.
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##### `buildPythonPackage` parameters {#buildpythonpackage-parameters}
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All parameters from [`stdenv.mkDerivation`](#sec-using-stdenv) function are still supported. The
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following are specific to `buildPythonPackage`:
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* `catchConflicts ? true`: If `true`, abort package build if a package name
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appears more than once in dependency tree. Default is `true`.
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* `disabled ? false`: If `true`, package is not built for the particular Python
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interpreter version.
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* `dontWrapPythonPrograms ? false`: Skip wrapping of Python programs.
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* `permitUserSite ? false`: Skip setting the `PYTHONNOUSERSITE` environment
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variable in wrapped programs.
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* `pyproject`: Whether the pyproject format should be used. As all other formats
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are deprecated, you are recommended to set this to `true`. When you do so,
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`pypaBuildHook` will be used, and you can add the required build dependencies
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from `build-system.requires` to `build-system`. Note that the pyproject
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format falls back to using `setuptools`, so you can use `pyproject = true`
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even if the package only has a `setup.py`. When set to `false`, you can
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use the existing [hooks](#setup-hooks) or provide your own logic to build the
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package. This can be useful for packages that don't support the pyproject
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format. When unset, the legacy `setuptools` hooks are used for backwards
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compatibility.
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* `makeWrapperArgs ? []`: A list of strings. Arguments to be passed to
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[`makeWrapper`](#fun-makeWrapper), which wraps generated binaries. By default, the arguments to
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[`makeWrapper`](#fun-makeWrapper) set `PATH` and `PYTHONPATH` environment variables before calling
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the binary. Additional arguments here can allow a developer to set environment
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variables which will be available when the binary is run. For example,
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`makeWrapperArgs = ["--set FOO BAR" "--set BAZ QUX"]`.
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* `namePrefix`: Prepends text to `${name}` parameter. In case of libraries, this
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defaults to `"python3.8-"` for Python 3.8, etc., and in case of applications to `""`.
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* `pypaBuildFlags ? []`: A list of strings. Arguments to be passed to `python -m build --wheel`.
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* `pythonPath ? []`: List of packages to be added into `$PYTHONPATH`. Packages
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in `pythonPath` are not propagated (contrary to [`propagatedBuildInputs`](#var-stdenv-propagatedBuildInputs)).
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* `preShellHook`: Hook to execute commands before `shellHook`.
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* `postShellHook`: Hook to execute commands after `shellHook`.
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* `removeBinByteCode ? true`: Remove bytecode from `/bin`. Bytecode is only
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created when the filenames end with `.py`.
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* `setupPyGlobalFlags ? []`: List of flags passed to `setup.py` command.
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* `setupPyBuildFlags ? []`: List of flags passed to `setup.py build_ext` command.
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The [`stdenv.mkDerivation`](#sec-using-stdenv) function accepts various parameters for describing
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build inputs (see "Specifying dependencies"). The following are of special
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interest for Python packages, either because these are primarily used, or
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because their behaviour is different:
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* `nativeBuildInputs ? []`: Build-time only dependencies. Typically executables.
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* `build-system ? []`: Build-time only Python dependencies. Items listed in `build-system.requires`/`setup_requires`.
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* `buildInputs ? []`: Build and/or run-time dependencies that need to be
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compiled for the host machine. Typically non-Python libraries which are being
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linked.
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* `nativeCheckInputs ? []`: Dependencies needed for running the [`checkPhase`](#ssec-check-phase). These
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are added to [`nativeBuildInputs`](#var-stdenv-nativeBuildInputs) when [`doCheck = true`](#var-stdenv-doCheck). Items listed in
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`tests_require` go here.
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* `dependencies ? []`: Aside from propagating dependencies,
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`buildPythonPackage` also injects code into and wraps executables with the
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paths included in this list. Items listed in `install_requires` go here.
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* `optional-dependencies ? { }`: Optional feature flagged dependencies. Items listed in `extras_requires` go here.
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##### Overriding Python packages {#overriding-python-packages}
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The `buildPythonPackage` function has a `overridePythonAttrs` method that can be
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used to override the package. In the following example we create an environment
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where we have the `blaze` package using an older version of `pandas`. We
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override first the Python interpreter and pass `packageOverrides` which contains
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the overrides for packages in the package set.
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```nix
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with import <nixpkgs> {};
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(let
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python = let
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packageOverrides = self: super: {
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pandas = super.pandas.overridePythonAttrs(old: rec {
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version = "0.19.1";
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src = fetchPypi {
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pname = "pandas";
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inherit version;
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hash = "sha256-JQn+rtpy/OA2deLszSKEuxyttqBzcAil50H+JDHUdCE=";
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};
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});
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};
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in pkgs.python3.override {inherit packageOverrides; self = python;};
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in python.withPackages(ps: [ ps.blaze ])).env
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```
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The next example shows a non trivial overriding of the `blas` implementation to
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be used through out all of the Python package set:
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```nix
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{
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python3MyBlas = pkgs.python3.override {
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packageOverrides = self: super: {
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# We need toPythonModule for the package set to evaluate this
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blas = super.toPythonModule(super.pkgs.blas.override {
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blasProvider = super.pkgs.mkl;
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});
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lapack = super.toPythonModule(super.pkgs.lapack.override {
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lapackProvider = super.pkgs.mkl;
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});
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};
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};
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}
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```
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This is particularly useful for numpy and scipy users who want to gain speed with other blas implementations.
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Note that using `scipy = super.scipy.override { blas = super.pkgs.mkl; };` will likely result in
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compilation issues, because scipy dependencies need to use the same blas implementation as well.
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#### `buildPythonApplication` function {#buildpythonapplication-function}
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The [`buildPythonApplication`](#buildpythonapplication-function) function is practically the same as
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[`buildPythonPackage`](#buildpythonpackage-function). The main purpose of this function is to build a Python
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package where one is interested only in the executables, and not importable
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modules. For that reason, when adding this package to a [`python.buildEnv`](#python.buildenv-function), the
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modules won't be made available.
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Another difference is that [`buildPythonPackage`](#buildpythonpackage-function) by default prefixes the names of
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the packages with the version of the interpreter. Because this is irrelevant for
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applications, the prefix is omitted.
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When packaging a Python application with [`buildPythonApplication`](#buildpythonapplication-function), it should be
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called with `callPackage` and passed `python3` or `python3Packages` (possibly
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specifying an interpreter version), like this:
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```nix
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{ lib
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, python3Packages
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, fetchPypi
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}:
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python3Packages.buildPythonApplication rec {
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pname = "luigi";
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version = "2.7.9";
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pyproject = true;
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src = fetchPypi {
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inherit pname version;
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hash = "sha256-Pe229rT0aHwA98s+nTHQMEFKZPo/yw6sot8MivFDvAw=";
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};
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build-system = with python3Packages; [
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setuptools
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];
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dependencies = with python3Packages; [
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tornado
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python-daemon
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];
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meta = {
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# ...
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};
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}
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```
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This is then added to `pkgs/by-name` just as any other application would be.
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Since the package is an application, a consumer doesn't need to care about
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Python versions or modules, which is why they don't go in `python3Packages`.
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#### `toPythonApplication` function {#topythonapplication-function}
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A distinction is made between applications and libraries, however, sometimes a
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package is used as both. In this case the package is added as a library to
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`python-packages.nix` and as an application to `pkgs/by-name`. To reduce
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duplication the `toPythonApplication` can be used to convert a library to an
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application.
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The Nix expression shall use [`buildPythonPackage`](#buildpythonpackage-function) and be called from
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`python-packages.nix`. A reference shall be created from `pkgs/by-name` to
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the attribute in `python-packages.nix`, and the `toPythonApplication` shall be
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applied to the reference:
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```nix
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{
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python3Packages,
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}:
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python3Packages.toPythonApplication python3Packages.youtube-dl
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```
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#### `toPythonModule` function {#topythonmodule-function}
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In some cases, such as bindings, a package is created using
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[`stdenv.mkDerivation`](#sec-using-stdenv) and added as attribute in `pkgs/by-name` or in `all-packages.nix`. The Python
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bindings should be made available from `python-packages.nix`. The
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`toPythonModule` function takes a derivation and makes certain Python-specific
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modifications.
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```nix
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{
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opencv = toPythonModule (pkgs.opencv.override {
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enablePython = true;
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pythonPackages = self;
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});
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}
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```
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Do pay attention to passing in the right Python version!
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#### `mkPythonMetaPackage` function {#mkpythonmetapackage-function}
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This will create a meta package containing [metadata files](https://packaging.python.org/en/latest/specifications/recording-installed-packages/) to satisfy a dependency on a package, without it actually having been installed into the environment.
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In nixpkgs this is used to package Python packages with split binary/source distributions such as [psycopg2](https://pypi.org/project/psycopg2/)/[psycopg2-binary](https://pypi.org/project/psycopg2-binary/).
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```nix
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mkPythonMetaPackage {
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pname = "psycopg2-binary";
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inherit (psycopg2) optional-dependencies version;
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dependencies = [ psycopg2 ];
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meta = {
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inherit (psycopg2.meta) description homepage;
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};
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}
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```
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#### `mkPythonEditablePackage` function {#mkpythoneditablepackage-function}
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When developing Python packages it's common to install packages in [editable mode](https://setuptools.pypa.io/en/latest/userguide/development_mode.html).
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Like `mkPythonMetaPackage` this function exists to create an otherwise empty package, but also containing a pointer to an impure location outside the Nix store that can be changed without rebuilding.
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The editable root is passed as a string. Normally `.pth` files contains absolute paths to the mutable location. This isn't always ergonomic with Nix, so environment variables are expanded at runtime.
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This means that a shell hook setting up something like a `$REPO_ROOT` variable can be used as the relative package root.
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As an implementation detail, the [PEP-518](https://peps.python.org/pep-0518/) `build-system` specified won't be used, but instead the editable package will be built using [hatchling](https://pypi.org/project/hatchling/).
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The `build-system`'s provided will instead become runtime dependencies of the editable package.
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Note that overriding packages deeper in the dependency graph _can_ work, but it's not the primary use case and overriding existing packages can make others break in unexpected ways.
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|
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``` nix
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{ pkgs ? import <nixpkgs> { } }:
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let
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pyproject = pkgs.lib.importTOML ./pyproject.toml;
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|
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myPython = pkgs.python.override {
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self = myPython;
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packageOverrides = pyfinal: pyprev: {
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# An editable package with a script that loads our mutable location
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my-editable = pyfinal.mkPythonEditablePackage {
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# Inherit project metadata from pyproject.toml
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pname = pyproject.project.name;
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inherit (pyproject.project) version;
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# The editable root passed as a string
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root = "$REPO_ROOT/src"; # Use environment variable expansion at runtime
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# Inject a script (other PEP-621 entrypoints are also accepted)
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inherit (pyproject.project) scripts;
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};
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};
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};
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pythonEnv = myPython.withPackages (ps: [ ps.my-editable ]);
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in pkgs.mkShell {
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packages = [ pythonEnv ];
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}
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```
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#### `python.buildEnv` function {#python.buildenv-function}
|
||
|
||
Python environments can be created using the low-level `pkgs.buildEnv` function.
|
||
This example shows how to create an environment that has the Pyramid Web Framework.
|
||
Saving the following as `default.nix`
|
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|
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```nix
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with import <nixpkgs> {};
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python3.buildEnv.override {
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extraLibs = [ python3Packages.pyramid ];
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ignoreCollisions = true;
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}
|
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```
|
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|
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and running `nix-build` will create
|
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|
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```
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/nix/store/cf1xhjwzmdki7fasgr4kz6di72ykicl5-python-2.7.8-env
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```
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with wrapped binaries in `bin/`.
|
||
|
||
You can also use the `env` attribute to create local environments with needed
|
||
packages installed. This is somewhat comparable to `virtualenv`. For example,
|
||
running `nix-shell` with the following `shell.nix`
|
||
|
||
```nix
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with import <nixpkgs> {};
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|
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(python3.buildEnv.override {
|
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extraLibs = with python3Packages; [
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numpy
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requests
|
||
];
|
||
}).env
|
||
```
|
||
|
||
will drop you into a shell where Python will have the
|
||
specified packages in its path.
|
||
|
||
##### `python.buildEnv` arguments {#python.buildenv-arguments}
|
||
|
||
|
||
* `extraLibs`: List of packages installed inside the environment.
|
||
* `postBuild`: Shell command executed after the build of environment.
|
||
* `ignoreCollisions`: Ignore file collisions inside the environment (default is `false`).
|
||
* `permitUserSite`: Skip setting the `PYTHONNOUSERSITE` environment variable in
|
||
wrapped binaries in the environment.
|
||
|
||
#### `python.withPackages` function {#python.withpackages-function}
|
||
|
||
The [`python.withPackages`](#python.withpackages-function) function provides a simpler interface to the [`python.buildEnv`](#python.buildenv-function) functionality.
|
||
It takes a function as an argument that is passed the set of python packages and returns the list
|
||
of the packages to be included in the environment. Using the [`withPackages`](#python.withpackages-function) function, the previous
|
||
example for the Pyramid Web Framework environment can be written like this:
|
||
|
||
```nix
|
||
with import <nixpkgs> {};
|
||
|
||
python.withPackages (ps: [ ps.pyramid ])
|
||
```
|
||
|
||
[`withPackages`](#python.withpackages-function) passes the correct package set for the specific interpreter
|
||
version as an argument to the function. In the above example, `ps` equals
|
||
`pythonPackages`. But you can also easily switch to using python3:
|
||
|
||
```nix
|
||
with import <nixpkgs> {};
|
||
|
||
python3.withPackages (ps: [ ps.pyramid ])
|
||
```
|
||
|
||
Now, `ps` is set to `python3Packages`, matching the version of the interpreter.
|
||
|
||
As [`python.withPackages`](#python.withpackages-function) uses [`python.buildEnv`](#python.buildenv-function) under the hood, it also
|
||
supports the `env` attribute. The `shell.nix` file from the previous section can
|
||
thus be also written like this:
|
||
|
||
```nix
|
||
with import <nixpkgs> {};
|
||
|
||
(python3.withPackages (ps: with ps; [
|
||
numpy
|
||
requests
|
||
])).env
|
||
```
|
||
|
||
In contrast to [`python.buildEnv`](#python.buildenv-function), [`python.withPackages`](#python.withpackages-function) does not support the
|
||
more advanced options such as `ignoreCollisions = true` or `postBuild`. If you
|
||
need them, you have to use [`python.buildEnv`](#python.buildenv-function).
|
||
|
||
Python 2 namespace packages may provide `__init__.py` that collide. In that case
|
||
[`python.buildEnv`](#python.buildenv-function) should be used with `ignoreCollisions = true`.
|
||
|
||
#### Setup hooks {#setup-hooks}
|
||
|
||
The following are setup hooks specifically for Python packages. Most of these
|
||
are used in [`buildPythonPackage`](#buildpythonpackage-function).
|
||
|
||
- `eggUnpackhook` to move an egg to the correct folder so it can be installed
|
||
with the `eggInstallHook`
|
||
- `eggBuildHook` to skip building for eggs.
|
||
- `eggInstallHook` to install eggs.
|
||
- `pypaBuildHook` to build a wheel using
|
||
[`pypa/build`](https://pypa-build.readthedocs.io/en/latest/index.html) and
|
||
PEP 517/518. Note a build system (e.g. `setuptools` or `flit`) should still
|
||
be added as `build-system`.
|
||
- `pypaInstallHook` to install wheels.
|
||
- `pytestCheckHook` to run tests with `pytest`. See [example usage](#using-pytestcheckhook).
|
||
- `pythonCatchConflictsHook` to fail if the package depends on two different versions of the same dependency.
|
||
- `pythonImportsCheckHook` to check whether importing the listed modules works.
|
||
- `pythonRelaxDepsHook` will relax Python dependencies restrictions for the package.
|
||
See [example usage](#using-pythonrelaxdepshook).
|
||
- `pythonRemoveBinBytecode` to remove bytecode from the `/bin` folder.
|
||
- `setuptoolsBuildHook` to build a wheel using `setuptools`.
|
||
- `sphinxHook` to build documentation and manpages using Sphinx.
|
||
- `venvShellHook` to source a Python 3 `venv` at the `venvDir` location. A
|
||
`venv` is created if it does not yet exist. `postVenvCreation` can be used to
|
||
to run commands only after venv is first created.
|
||
- `wheelUnpackHook` to move a wheel to the correct folder so it can be installed
|
||
with the `pipInstallHook`.
|
||
- `unittestCheckHook` will run tests with `python -m unittest discover`. See [example usage](#using-unittestcheckhook).
|
||
|
||
## User Guide {#user-guide}
|
||
|
||
### Using Python {#using-python}
|
||
|
||
#### Overview {#overview}
|
||
|
||
Several versions of the Python interpreter are available on Nix, as well as a
|
||
high amount of packages. The attribute `python3` refers to the default
|
||
interpreter, which is currently CPython 3.12. The attribute `python` refers to
|
||
CPython 2.7 for backwards-compatibility. It is also possible to refer to
|
||
specific versions, e.g. `python312` refers to CPython 3.12, and `pypy` refers to
|
||
the default PyPy interpreter.
|
||
|
||
Python is used a lot, and in different ways. This affects also how it is
|
||
packaged. In the case of Python on Nix, an important distinction is made between
|
||
whether the package is considered primarily an application, or whether it should
|
||
be used as a library, i.e., of primary interest are the modules in
|
||
`site-packages` that should be importable.
|
||
|
||
In the Nixpkgs tree Python applications can be found throughout, depending on
|
||
what they do, and are called from the main package set. Python libraries,
|
||
however, are in separate sets, with one set per interpreter version.
|
||
|
||
The interpreters have several common attributes. One of these attributes is
|
||
`pkgs`, which is a package set of Python libraries for this specific
|
||
interpreter. E.g., the `toolz` package corresponding to the default interpreter
|
||
is `python3.pkgs.toolz`, and the CPython 3.12 version is `python312.pkgs.toolz`.
|
||
The main package set contains aliases to these package sets, e.g.
|
||
`pythonPackages` refers to `python.pkgs` and `python312Packages` to
|
||
`python312.pkgs`.
|
||
|
||
#### Installing Python and packages {#installing-python-and-packages}
|
||
|
||
The Nix and NixOS manuals explain how packages are generally installed. In the
|
||
case of Python and Nix, it is important to make a distinction between whether the
|
||
package is considered an application or a library.
|
||
|
||
Applications on Nix are typically installed into your user profile imperatively
|
||
using `nix-env -i`, and on NixOS declaratively by adding the package name to
|
||
`environment.systemPackages` in `/etc/nixos/configuration.nix`. Dependencies
|
||
such as libraries are automatically installed and should not be installed
|
||
explicitly.
|
||
|
||
The same goes for Python applications. Python applications can be installed in
|
||
your profile, and will be wrapped to find their exact library dependencies,
|
||
without impacting other applications or polluting your user environment.
|
||
|
||
But Python libraries you would like to use for development cannot be installed,
|
||
at least not individually, because they won't be able to find each other
|
||
resulting in import errors. Instead, it is possible to create an environment
|
||
with [`python.buildEnv`](#python.buildenv-function) or [`python.withPackages`](#python.withpackages-function) where the interpreter and other
|
||
executables are wrapped to be able to find each other and all of the modules.
|
||
|
||
In the following examples we will start by creating a simple, ad-hoc environment
|
||
with a nix-shell that has `numpy` and `toolz` in Python 3.12; then we will create
|
||
a re-usable environment in a single-file Python script; then we will create a
|
||
full Python environment for development with this same environment.
|
||
|
||
Philosophically, this should be familiar to users who are used to a `venv` style
|
||
of development: individual projects create their own Python environments without
|
||
impacting the global environment or each other.
|
||
|
||
#### Ad-hoc temporary Python environment with `nix-shell` {#ad-hoc-temporary-python-environment-with-nix-shell}
|
||
|
||
The simplest way to start playing with the way nix wraps and sets up Python
|
||
environments is with `nix-shell` at the cmdline. These environments create a
|
||
temporary shell session with a Python and a *precise* list of packages (plus
|
||
their runtime dependencies), with no other Python packages in the Python
|
||
interpreter's scope.
|
||
|
||
To create a Python 3.12 session with `numpy` and `toolz` available, run:
|
||
|
||
```sh
|
||
$ nix-shell -p 'python312.withPackages(ps: with ps; [ numpy toolz ])'
|
||
```
|
||
|
||
By default `nix-shell` will start a `bash` session with this interpreter in our
|
||
`PATH`, so if we then run:
|
||
|
||
```Python console
|
||
[nix-shell:~/src/nixpkgs]$ python3
|
||
Python 3.12.4 (main, Jun 6 2024, 18:26:44) [GCC 13.3.0] on linux
|
||
Type "help", "copyright", "credits" or "license" for more information.
|
||
>>> import numpy; import toolz
|
||
```
|
||
|
||
Note that no other modules are in scope, even if they were imperatively
|
||
installed into our user environment as a dependency of a Python application:
|
||
|
||
```Python console
|
||
>>> import requests
|
||
Traceback (most recent call last):
|
||
File "<stdin>", line 1, in <module>
|
||
ModuleNotFoundError: No module named 'requests'
|
||
```
|
||
|
||
We can add as many additional modules onto the `nix-shell` as we need, and we
|
||
will still get 1 wrapped Python interpreter. We can start the interpreter
|
||
directly like so:
|
||
|
||
```sh
|
||
$ nix-shell -p "python312.withPackages (ps: with ps; [ numpy toolz requests ])" --run python3
|
||
Python 3.12.4 (main, Jun 6 2024, 18:26:44) [GCC 13.3.0] on linux
|
||
Type "help", "copyright", "credits" or "license" for more information.
|
||
>>> import requests
|
||
>>>
|
||
```
|
||
|
||
Notice that this time it built a new Python environment, which now includes
|
||
`requests`. Building an environment just creates wrapper scripts that expose the
|
||
selected dependencies to the interpreter while re-using the actual modules. This
|
||
means if any other env has installed `requests` or `numpy` in a different
|
||
context, we don't need to recompile them -- we just recompile the wrapper script
|
||
that sets up an interpreter pointing to them. This matters much more for "big"
|
||
modules like `pytorch` or `tensorflow`.
|
||
|
||
Module names usually match their names on [pypi.org](https://pypi.org/), but
|
||
normalized according to PEP 503/508. (e.g. Foo__Bar.baz -> foo-bar-baz)
|
||
You can use the [Nixpkgs search website](https://nixos.org/nixos/packages.html)
|
||
to find them as well (along with non-python packages).
|
||
|
||
At this point we can create throwaway experimental Python environments with
|
||
arbitrary dependencies. This is a good way to get a feel for how the Python
|
||
interpreter and dependencies work in Nix and NixOS, but to do some actual
|
||
development, we'll want to make it a bit more persistent.
|
||
|
||
##### Running Python scripts and using `nix-shell` as shebang {#running-python-scripts-and-using-nix-shell-as-shebang}
|
||
|
||
Sometimes, we have a script whose header looks like this:
|
||
|
||
```python
|
||
#!/usr/bin/env python3
|
||
import numpy as np
|
||
a = np.array([1,2])
|
||
b = np.array([3,4])
|
||
print(f"The dot product of {a} and {b} is: {np.dot(a, b)}")
|
||
```
|
||
|
||
Executing this script requires a `python3` that has `numpy`. Using what we learned
|
||
in the previous section, we could startup a shell and just run it like so:
|
||
|
||
```ShellSession
|
||
$ nix-shell -p 'python312.withPackages (ps: with ps; [ numpy ])' --run 'python3 foo.py'
|
||
The dot product of [1 2] and [3 4] is: 11
|
||
```
|
||
|
||
But if we maintain the script ourselves, and if there are more dependencies, it
|
||
may be nice to encode those dependencies in source to make the script re-usable
|
||
without that bit of knowledge. That can be done by using `nix-shell` as a
|
||
[shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)), like so:
|
||
|
||
```python
|
||
#!/usr/bin/env nix-shell
|
||
#!nix-shell -i python3 -p "python3.withPackages(ps: [ ps.numpy ])"
|
||
import numpy as np
|
||
a = np.array([1,2])
|
||
b = np.array([3,4])
|
||
print(f"The dot product of {a} and {b} is: {np.dot(a, b)}")
|
||
```
|
||
|
||
Then we execute it, without requiring any environment setup at all!
|
||
|
||
```sh
|
||
$ ./foo.py
|
||
The dot product of [1 2] and [3 4] is: 11
|
||
```
|
||
|
||
If the dependencies are not available on the host where `foo.py` is executed, it
|
||
will build or download them from a Nix binary cache prior to starting up, prior
|
||
that it is executed on a machine with a multi-user nix installation.
|
||
|
||
This provides a way to ship a self bootstrapping Python script, akin to a
|
||
statically linked binary, where it can be run on any machine (provided nix is
|
||
installed) without having to assume that `numpy` is installed globally on the
|
||
system.
|
||
|
||
By default it is pulling the import checkout of Nixpkgs itself from our nix
|
||
channel, which is nice as it cache aligns with our other package builds, but we
|
||
can make it fully reproducible by pinning the `nixpkgs` import:
|
||
|
||
```python
|
||
#!/usr/bin/env nix-shell
|
||
#!nix-shell -i python3 -p "python3.withPackages (ps: [ ps.numpy ])"
|
||
#!nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/e51209796c4262bfb8908e3d6d72302fe4e96f5f.tar.gz
|
||
import numpy as np
|
||
a = np.array([1,2])
|
||
b = np.array([3,4])
|
||
print(f"The dot product of {a} and {b} is: {np.dot(a, b)}")
|
||
```
|
||
|
||
This will execute with the exact same versions of Python 3.10, numpy, and system
|
||
dependencies a year from now as it does today, because it will always use
|
||
exactly git commit `e51209796c4262bfb8908e3d6d72302fe4e96f5f` of Nixpkgs for all
|
||
of the package versions.
|
||
|
||
This is also a great way to ensure the script executes identically on different
|
||
servers.
|
||
|
||
##### Load environment from `.nix` expression {#load-environment-from-.nix-expression}
|
||
|
||
We've now seen how to create an ad-hoc temporary shell session, and how to
|
||
create a single script with Python dependencies, but in the course of normal
|
||
development we're usually working in an entire package repository.
|
||
|
||
As explained [in the `nix-shell` section](https://nixos.org/manual/nix/stable/command-ref/nix-shell) of the Nix manual, `nix-shell` can also load an expression from a `.nix` file.
|
||
Say we want to have Python 3.12, `numpy` and `toolz`, like before,
|
||
in an environment. We can add a `shell.nix` file describing our dependencies:
|
||
|
||
```nix
|
||
with import <nixpkgs> {};
|
||
(python312.withPackages (ps: with ps; [
|
||
numpy
|
||
toolz
|
||
])).env
|
||
```
|
||
|
||
And then at the command line, just typing `nix-shell` produces the same
|
||
environment as before. In a normal project, we'll likely have many more
|
||
dependencies; this can provide a way for developers to share the environments
|
||
with each other and with CI builders.
|
||
|
||
What's happening here?
|
||
|
||
1. We begin with importing the Nix Packages collections. `import <nixpkgs>`
|
||
imports the `<nixpkgs>` function, `{}` calls it and the `with` statement
|
||
brings all attributes of `nixpkgs` in the local scope. These attributes form
|
||
the main package set.
|
||
2. Then we create a Python 3.12 environment with the [`withPackages`](#python.withpackages-function) function, as before.
|
||
3. The [`withPackages`](#python.withpackages-function) function expects us to provide a function as an argument
|
||
that takes the set of all Python packages and returns a list of packages to
|
||
include in the environment. Here, we select the packages `numpy` and `toolz`
|
||
from the package set.
|
||
|
||
To combine this with `mkShell` you can:
|
||
|
||
```nix
|
||
with import <nixpkgs> {};
|
||
let
|
||
pythonEnv = python312.withPackages (ps: [
|
||
ps.numpy
|
||
ps.toolz
|
||
]);
|
||
in mkShell {
|
||
packages = [
|
||
pythonEnv
|
||
|
||
black
|
||
mypy
|
||
|
||
libffi
|
||
openssl
|
||
];
|
||
}
|
||
```
|
||
|
||
This will create a unified environment that has not just our Python interpreter
|
||
and its Python dependencies, but also tools like `black` or `mypy` and libraries
|
||
like `libffi` the `openssl` in scope. This is generic and can span any number of
|
||
tools or languages across the Nixpkgs ecosystem.
|
||
|
||
##### Installing environments globally on the system {#installing-environments-globally-on-the-system}
|
||
|
||
Up to now, we've been creating environments scoped to an ad-hoc shell session,
|
||
or a single script, or a single project. This is generally advisable, as it
|
||
avoids pollution across contexts.
|
||
|
||
However, sometimes we know we will often want a Python with some basic packages,
|
||
and want this available without having to enter into a shell or build context.
|
||
This can be useful to have things like vim/emacs editors and plugins or shell
|
||
tools "just work" without having to set them up, or when running other software
|
||
that expects packages to be installed globally.
|
||
|
||
To create your own custom environment, create a file in `~/.config/nixpkgs/overlays/`
|
||
that looks like this:
|
||
|
||
```nix
|
||
# ~/.config/nixpkgs/overlays/myEnv.nix
|
||
self: super: {
|
||
myEnv = super.buildEnv {
|
||
name = "myEnv";
|
||
paths = [
|
||
# A Python 3 interpreter with some packages
|
||
(self.python3.withPackages (
|
||
ps: with ps; [
|
||
pyflakes
|
||
pytest
|
||
black
|
||
]
|
||
))
|
||
|
||
# Some other packages we'd like as part of this env
|
||
self.mypy
|
||
self.black
|
||
self.ripgrep
|
||
self.tmux
|
||
];
|
||
};
|
||
}
|
||
```
|
||
|
||
You can then build and install this to your profile with:
|
||
|
||
```sh
|
||
nix-env -iA myEnv
|
||
```
|
||
|
||
One limitation of this is that you can only have 1 Python env installed
|
||
globally, since they conflict on the `python` to load out of your `PATH`.
|
||
|
||
If you get a conflict or prefer to keep the setup clean, you can have `nix-env`
|
||
atomically *uninstall* all other imperatively installed packages and replace
|
||
your profile with just `myEnv` by using the `--replace` flag.
|
||
|
||
##### Environment defined in `/etc/nixos/configuration.nix` {#environment-defined-in-etcnixosconfiguration.nix}
|
||
|
||
For the sake of completeness, here's how to install the environment system-wide
|
||
on NixOS.
|
||
|
||
```nix
|
||
{ # ...
|
||
|
||
environment.systemPackages = with pkgs; [
|
||
(python310.withPackages(ps: with ps; [ numpy toolz ]))
|
||
];
|
||
}
|
||
```
|
||
|
||
### Developing with Python {#developing-with-python}
|
||
|
||
Above, we were mostly just focused on use cases and what to do to get started
|
||
creating working Python environments in nix.
|
||
|
||
Now that you know the basics to be up and running, it is time to take a step
|
||
back and take a deeper look at how Python packages are packaged on Nix.
|
||
|
||
#### Python library packages in Nixpkgs {#python-library-packages-in-nixpkgs}
|
||
|
||
With Nix all packages are built by functions. The main function in Nix for
|
||
building Python libraries is [`buildPythonPackage`](#buildpythonpackage-function). Let's see how we can build the
|
||
`toolz` package.
|
||
|
||
```nix
|
||
{ lib
|
||
, buildPythonPackage
|
||
, fetchPypi
|
||
, setuptools
|
||
}:
|
||
|
||
buildPythonPackage rec {
|
||
pname = "toolz";
|
||
version = "0.10.0";
|
||
pyproject = true;
|
||
|
||
src = fetchPypi {
|
||
inherit pname version;
|
||
hash = "sha256-CP3V73yWSArRHBLUct4hrNMjWZlvaaUlkpm1QP66RWA=";
|
||
};
|
||
|
||
build-system = [
|
||
setuptools
|
||
];
|
||
|
||
# has no tests
|
||
doCheck = false;
|
||
|
||
pythonImportsCheck = [
|
||
"toolz.itertoolz"
|
||
"toolz.functoolz"
|
||
"toolz.dicttoolz"
|
||
];
|
||
|
||
meta = {
|
||
changelog = "https://github.com/pytoolz/toolz/releases/tag/${version}";
|
||
homepage = "https://github.com/pytoolz/toolz";
|
||
description = "List processing tools and functional utilities";
|
||
license = lib.licenses.bsd3;
|
||
};
|
||
}
|
||
```
|
||
|
||
What happens here? The function [`buildPythonPackage`](#buildpythonpackage-function) is called and as argument
|
||
it accepts a set. In this case the set is a recursive set, `rec`. One of the
|
||
arguments is the name of the package, which consists of a basename (generally
|
||
following the name on PyPI) and a version. Another argument, `src` specifies the
|
||
source, which in this case is fetched from PyPI using the helper function
|
||
`fetchPypi`. The argument `doCheck` is used to set whether tests should be run
|
||
when building the package. Since there are no tests, we rely on [`pythonImportsCheck`](#using-pythonimportscheck)
|
||
to test whether the package can be imported. Furthermore, we specify some meta
|
||
information. The output of the function is a derivation.
|
||
|
||
An expression for `toolz` can be found in the Nixpkgs repository. As explained
|
||
in the introduction of this Python section, a derivation of `toolz` is available
|
||
for each interpreter version, e.g. `python312.pkgs.toolz` refers to the `toolz`
|
||
derivation corresponding to the CPython 3.12 interpreter.
|
||
|
||
The above example works when you're directly working on
|
||
`pkgs/top-level/python-packages.nix` in the Nixpkgs repository. Often though,
|
||
you will want to test a Nix expression outside of the Nixpkgs tree.
|
||
|
||
The following expression creates a derivation for the `toolz` package,
|
||
and adds it along with a `numpy` package to a Python environment.
|
||
|
||
```nix
|
||
with import <nixpkgs> {};
|
||
|
||
( let
|
||
my_toolz = python312.pkgs.buildPythonPackage rec {
|
||
pname = "toolz";
|
||
version = "0.10.0";
|
||
pyproject = true;
|
||
|
||
src = fetchPypi {
|
||
inherit pname version;
|
||
hash = "sha256-CP3V73yWSArRHBLUct4hrNMjWZlvaaUlkpm1QP66RWA=";
|
||
};
|
||
|
||
build-system = [
|
||
python312.pkgs.setuptools
|
||
];
|
||
|
||
# has no tests
|
||
doCheck = false;
|
||
|
||
meta = {
|
||
homepage = "https://github.com/pytoolz/toolz/";
|
||
description = "List processing tools and functional utilities";
|
||
# [...]
|
||
};
|
||
};
|
||
|
||
in python312.withPackages (ps: with ps; [
|
||
numpy
|
||
my_toolz
|
||
])
|
||
).env
|
||
```
|
||
|
||
Executing `nix-shell` will result in an environment in which you can use
|
||
Python 3.12 and the `toolz` package. As you can see we had to explicitly mention
|
||
for which Python version we want to build a package.
|
||
|
||
So, what did we do here? Well, we took the Nix expression that we used earlier
|
||
to build a Python environment, and said that we wanted to include our own
|
||
version of `toolz`, named `my_toolz`. To introduce our own package in the scope
|
||
of [`withPackages`](#python.withpackages-function) we used a `let` expression. You can see that we used
|
||
`ps.numpy` to select numpy from the nixpkgs package set (`ps`). We did not take
|
||
`toolz` from the Nixpkgs package set this time, but instead took our own version
|
||
that we introduced with the `let` expression.
|
||
|
||
#### Handling dependencies {#handling-dependencies}
|
||
|
||
Our example, `toolz`, does not have any dependencies on other Python packages or system libraries.
|
||
[`buildPythonPackage`](#buildpythonpackage-function) uses the the following arguments in the following circumstances:
|
||
|
||
- `dependencies` - For Python runtime dependencies.
|
||
- `build-system` - For Python build-time requirements.
|
||
- [`buildInputs`](#var-stdenv-buildInputs) - For non-Python build-time requirements.
|
||
- [`nativeCheckInputs`](#var-stdenv-nativeCheckInputs) - For test dependencies
|
||
|
||
Dependencies can belong to multiple arguments, for example if something is both a build time requirement & a runtime dependency.
|
||
|
||
The following example shows which arguments are given to [`buildPythonPackage`](#buildpythonpackage-function) in
|
||
order to build [`datashape`](https://github.com/blaze/datashape).
|
||
|
||
```nix
|
||
{ lib
|
||
, buildPythonPackage
|
||
, fetchPypi
|
||
|
||
# build dependencies
|
||
, setuptools
|
||
|
||
# dependencies
|
||
, numpy, multipledispatch, python-dateutil
|
||
|
||
# tests
|
||
, pytestCheckHook
|
||
}:
|
||
|
||
buildPythonPackage rec {
|
||
pname = "datashape";
|
||
version = "0.4.7";
|
||
pyproject = true;
|
||
|
||
src = fetchPypi {
|
||
inherit pname version;
|
||
hash = "sha256-FLLvdm1MllKrgTGC6Gb0k0deZeVYvtCCLji/B7uhong=";
|
||
};
|
||
|
||
build-system = [
|
||
setuptools
|
||
];
|
||
|
||
dependencies = [
|
||
multipledispatch
|
||
numpy
|
||
python-dateutil
|
||
];
|
||
|
||
nativeCheckInputs = [
|
||
pytestCheckHook
|
||
];
|
||
|
||
meta = {
|
||
changelog = "https://github.com/blaze/datashape/releases/tag/${version}";
|
||
homepage = "https://github.com/ContinuumIO/datashape";
|
||
description = "Data description language";
|
||
license = lib.licenses.bsd2;
|
||
};
|
||
}
|
||
```
|
||
|
||
We can see several runtime dependencies, `numpy`, `multipledispatch`, and
|
||
`python-dateutil`. Furthermore, we have [`nativeCheckInputs`](#var-stdenv-nativeCheckInputs) with `pytestCheckHook`.
|
||
`pytestCheckHook` is a test runner hook and is only used during the [`checkPhase`](#ssec-check-phase) and is
|
||
therefore not added to `dependencies`.
|
||
|
||
In the previous case we had only dependencies on other Python packages to consider.
|
||
Occasionally you have also system libraries to consider. E.g., `lxml` provides
|
||
Python bindings to `libxml2` and `libxslt`. These libraries are only required
|
||
when building the bindings and are therefore added as [`buildInputs`](#var-stdenv-buildInputs).
|
||
|
||
```nix
|
||
{ lib
|
||
, buildPythonPackage
|
||
, fetchPypi
|
||
, setuptools
|
||
, libxml2
|
||
, libxslt
|
||
}:
|
||
|
||
buildPythonPackage rec {
|
||
pname = "lxml";
|
||
version = "3.4.4";
|
||
pyproject = true;
|
||
|
||
src = fetchPypi {
|
||
inherit pname version;
|
||
hash = "sha256-s9NiusRxFydHzaNRMjjxFcvWxfi45jGb9ql6eJJyQJk=";
|
||
};
|
||
|
||
build-system = [
|
||
setuptools
|
||
];
|
||
|
||
buildInputs = [
|
||
libxml2
|
||
libxslt
|
||
];
|
||
|
||
# tests are meant to be ran "in-place" in the same directory as src
|
||
doCheck = false;
|
||
|
||
pythonImportsCheck = [
|
||
"lxml"
|
||
"lxml.etree"
|
||
];
|
||
|
||
meta = {
|
||
changelog = "https://github.com/lxml/lxml/releases/tag/lxml-${version}";
|
||
description = "Pythonic binding for the libxml2 and libxslt libraries";
|
||
homepage = "https://lxml.de";
|
||
license = lib.licenses.bsd3;
|
||
maintainers = with lib.maintainers; [ sjourdois ];
|
||
};
|
||
}
|
||
```
|
||
|
||
In this example `lxml` and Nix are able to work out exactly where the relevant
|
||
files of the dependencies are. This is not always the case.
|
||
|
||
The example below shows bindings to The Fastest Fourier Transform in the West,
|
||
commonly known as FFTW. On Nix we have separate packages of FFTW for the
|
||
different types of floats (`"single"`, `"double"`, `"long-double"`). The
|
||
bindings need all three types, and therefore we add all three as [`buildInputs`](#var-stdenv-buildInputs).
|
||
The bindings don't expect to find each of them in a different folder, and
|
||
therefore we have to set `LDFLAGS` and `CFLAGS`.
|
||
|
||
```nix
|
||
{ lib
|
||
, buildPythonPackage
|
||
, fetchPypi
|
||
|
||
# build dependencies
|
||
, setuptools
|
||
|
||
# dependencies
|
||
, fftw
|
||
, fftwFloat
|
||
, fftwLongDouble
|
||
, numpy
|
||
, scipy
|
||
}:
|
||
|
||
buildPythonPackage rec {
|
||
pname = "pyfftw";
|
||
version = "0.9.2";
|
||
pyproject = true;
|
||
|
||
src = fetchPypi {
|
||
inherit pname version;
|
||
hash = "sha256-9ru2r6kwhUCaskiFoaPNuJCfCVoUL01J40byvRt4kHQ=";
|
||
};
|
||
|
||
build-system = [
|
||
setuptools
|
||
];
|
||
|
||
buildInputs = [
|
||
fftw
|
||
fftwFloat
|
||
fftwLongDouble
|
||
];
|
||
|
||
dependencies = [
|
||
numpy
|
||
scipy
|
||
];
|
||
|
||
preConfigure = ''
|
||
export LDFLAGS="-L${fftw.dev}/lib -L${fftwFloat.out}/lib -L${fftwLongDouble.out}/lib"
|
||
export CFLAGS="-I${fftw.dev}/include -I${fftwFloat.dev}/include -I${fftwLongDouble.dev}/include"
|
||
'';
|
||
|
||
# Tests cannot import pyfftw. pyfftw works fine though.
|
||
doCheck = false;
|
||
|
||
pythonImportsCheck = [ "pyfftw" ];
|
||
|
||
meta = {
|
||
changelog = "https://github.com/pyFFTW/pyFFTW/releases/tag/v${version}";
|
||
description = "Pythonic wrapper around FFTW, the FFT library, presenting a unified interface for all the supported transforms";
|
||
homepage = "http://hgomersall.github.com/pyFFTW";
|
||
license = with lib.licenses; [ bsd2 bsd3 ];
|
||
};
|
||
}
|
||
```
|
||
|
||
Note also the line [`doCheck = false;`](#var-stdenv-doCheck), we explicitly disabled running the test-suite.
|
||
|
||
#### Testing Python Packages {#testing-python-packages}
|
||
|
||
It is highly encouraged to have testing as part of the package build. This
|
||
helps to avoid situations where the package was able to build and install,
|
||
but is not usable at runtime.
|
||
Your package should provide its own [`checkPhase`](#ssec-check-phase).
|
||
|
||
::: {.note}
|
||
The [`checkPhase`](#ssec-check-phase) for python maps to the `installCheckPhase` on a
|
||
normal derivation. This is due to many python packages not behaving well
|
||
to the pre-installed version of the package. Version info, and natively
|
||
compiled extensions generally only exist in the install directory, and
|
||
thus can cause issues when a test suite asserts on that behavior.
|
||
:::
|
||
|
||
::: {.note}
|
||
Tests should only be disabled if they don't agree with nix
|
||
(e.g. external dependencies, network access, flakey tests), however,
|
||
as many tests should be enabled as possible. Failing tests can still be
|
||
a good indication that the package is not in a valid state.
|
||
:::
|
||
|
||
#### Using pytest {#using-pytest}
|
||
|
||
Pytest is the most common test runner for python repositories. A trivial
|
||
test run would be:
|
||
|
||
```nix
|
||
{
|
||
nativeCheckInputs = [ pytest ];
|
||
checkPhase = ''
|
||
runHook preCheck
|
||
|
||
pytest
|
||
|
||
runHook postCheck
|
||
'';
|
||
}
|
||
```
|
||
|
||
However, many repositories' test suites do not translate well to nix's build
|
||
sandbox, and will generally need many tests to be disabled.
|
||
|
||
To filter tests using pytest, one can do the following:
|
||
|
||
```nix
|
||
{
|
||
nativeCheckInputs = [ pytest ];
|
||
# avoid tests which need additional data or touch network
|
||
checkPhase = ''
|
||
runHook preCheck
|
||
|
||
pytest tests/ --ignore=tests/integration -k 'not download and not update' --ignore=tests/test_failing.py
|
||
|
||
runHook postCheck
|
||
'';
|
||
}
|
||
```
|
||
|
||
`--ignore` will tell pytest to ignore that file or directory from being
|
||
collected as part of a test run. This is useful is a file uses a package
|
||
which is not available in nixpkgs, thus skipping that test file is much
|
||
easier than having to create a new package.
|
||
|
||
`-k` is used to define a predicate for test names. In this example, we are
|
||
filtering out tests which contain `download` or `update` in their test case name.
|
||
Only one `-k` argument is allowed, and thus a long predicate should be concatenated
|
||
with “\\” and wrapped to the next line.
|
||
|
||
::: {.note}
|
||
In pytest==6.0.1, the use of “\\” to continue a line (e.g. `-k 'not download \'`) has
|
||
been removed, in this case, it's recommended to use `pytestCheckHook`.
|
||
:::
|
||
|
||
#### Using pytestCheckHook {#using-pytestcheckhook}
|
||
|
||
`pytestCheckHook` is a convenient hook which will set up (or configure)
|
||
a [`checkPhase`](#ssec-check-phase) to run `pytest`. This is also beneficial
|
||
when a package may need many items disabled to run the test suite.
|
||
Most packages use `pytest` or `unittest`, which is compatible with `pytest`,
|
||
so you will most likely use `pytestCheckHook`.
|
||
|
||
Using the example above, the analogous `pytestCheckHook` usage would be:
|
||
|
||
```nix
|
||
{
|
||
nativeCheckInputs = [
|
||
pytestCheckHook
|
||
];
|
||
|
||
# requires additional data
|
||
pytestFlagsArray = [
|
||
"tests/"
|
||
"--ignore=tests/integration"
|
||
];
|
||
|
||
disabledTests = [
|
||
# touches network
|
||
"download"
|
||
"update"
|
||
];
|
||
|
||
disabledTestPaths = [
|
||
"tests/test_failing.py"
|
||
];
|
||
}
|
||
```
|
||
|
||
This is especially useful when tests need to be conditionally disabled,
|
||
for example:
|
||
|
||
```nix
|
||
{
|
||
disabledTests = [
|
||
# touches network
|
||
"download"
|
||
"update"
|
||
] ++ lib.optionals (pythonAtLeast "3.8") [
|
||
# broken due to python3.8 async changes
|
||
"async"
|
||
] ++ lib.optionals stdenv.buildPlatform.isDarwin [
|
||
# can fail when building with other packages
|
||
"socket"
|
||
];
|
||
}
|
||
```
|
||
|
||
Trying to concatenate the related strings to disable tests in a regular
|
||
[`checkPhase`](#ssec-check-phase) would be much harder to read. This also enables us to comment on
|
||
why specific tests are disabled.
|
||
|
||
#### Using pythonImportsCheck {#using-pythonimportscheck}
|
||
|
||
Although unit tests are highly preferred to validate correctness of a package, not
|
||
all packages have test suites that can be run easily, and some have none at all.
|
||
To help ensure the package still works, [`pythonImportsCheck`](#using-pythonimportscheck) can attempt to import
|
||
the listed modules.
|
||
|
||
```nix
|
||
{
|
||
pythonImportsCheck = [
|
||
"requests"
|
||
"urllib"
|
||
];
|
||
}
|
||
```
|
||
|
||
roughly translates to:
|
||
|
||
```nix
|
||
{
|
||
postCheck = ''
|
||
PYTHONPATH=$out/${python.sitePackages}:$PYTHONPATH
|
||
python -c "import requests; import urllib"
|
||
'';
|
||
}
|
||
```
|
||
|
||
However, this is done in its own phase, and not dependent on whether [`doCheck = true;`](#var-stdenv-doCheck).
|
||
|
||
This can also be useful in verifying that the package doesn't assume commonly
|
||
present packages (e.g. `setuptools`).
|
||
|
||
#### Using pythonRelaxDepsHook {#using-pythonrelaxdepshook}
|
||
|
||
It is common for upstream to specify a range of versions for its package
|
||
dependencies. This makes sense, since it ensures that the package will be built
|
||
with a subset of packages that is well tested. However, this commonly causes
|
||
issues when packaging in Nixpkgs, because the dependencies that this package
|
||
may need are too new or old for the package to build correctly. We also cannot
|
||
package multiple versions of the same package since this may cause conflicts
|
||
in `PYTHONPATH`.
|
||
|
||
One way to side step this issue is to relax the dependencies. This can be done
|
||
by either removing the package version range or by removing the package
|
||
declaration entirely. This can be done using the `pythonRelaxDepsHook` hook. For
|
||
example, given the following `requirements.txt` file:
|
||
|
||
```
|
||
pkg1<1.0
|
||
pkg2
|
||
pkg3>=1.0,<=2.0
|
||
```
|
||
|
||
we can do:
|
||
|
||
```nix
|
||
{
|
||
pythonRelaxDeps = [
|
||
"pkg1"
|
||
"pkg3"
|
||
];
|
||
pythonRemoveDeps = [
|
||
"pkg2"
|
||
];
|
||
}
|
||
```
|
||
|
||
which would result in the following `requirements.txt` file:
|
||
|
||
```
|
||
pkg1
|
||
pkg3
|
||
```
|
||
|
||
Another option is to pass `true`, that will relax/remove all dependencies, for
|
||
example:
|
||
|
||
```nix
|
||
{
|
||
pythonRelaxDeps = true;
|
||
}
|
||
```
|
||
|
||
which would result in the following `requirements.txt` file:
|
||
|
||
```
|
||
pkg1
|
||
pkg2
|
||
pkg3
|
||
```
|
||
|
||
In general you should always use `pythonRelaxDeps`, because `pythonRemoveDeps`
|
||
will convert build errors into runtime errors. However `pythonRemoveDeps` may
|
||
still be useful in exceptional cases, and also to remove dependencies wrongly
|
||
declared by upstream (for example, declaring `black` as a runtime dependency
|
||
instead of a dev dependency).
|
||
|
||
Keep in mind that while the examples above are done with `requirements.txt`,
|
||
`pythonRelaxDepsHook` works by modifying the resulting wheel file, so it should
|
||
work with any of the [existing hooks](#setup-hooks).
|
||
|
||
The `pythonRelaxDepsHook` has no effect on build time dependencies, such as
|
||
those specified in `build-system`. If a package requires incompatible build
|
||
time dependencies, they should be removed in `postPatch` through
|
||
`substituteInPlace` or similar.
|
||
|
||
For ease of use, both `buildPythonPackage` and `buildPythonApplication` will
|
||
automatically add `pythonRelaxDepsHook` if either `pythonRelaxDeps` or
|
||
`pythonRemoveDeps` is specified.
|
||
|
||
#### Using unittestCheckHook {#using-unittestcheckhook}
|
||
|
||
`unittestCheckHook` is a hook which will set up (or configure) a [`checkPhase`](#ssec-check-phase) to run `python -m unittest discover`:
|
||
|
||
```nix
|
||
{
|
||
nativeCheckInputs = [
|
||
unittestCheckHook
|
||
];
|
||
|
||
unittestFlagsArray = [
|
||
"-s" "tests" "-v"
|
||
];
|
||
}
|
||
```
|
||
|
||
`pytest` is compatible with `unittest`, so in most cases you can use `pytestCheckHook` instead.
|
||
|
||
#### Using sphinxHook {#using-sphinxhook}
|
||
|
||
The `sphinxHook` is a helpful tool to build documentation and manpages
|
||
using the popular Sphinx documentation generator.
|
||
It is setup to automatically find common documentation source paths and
|
||
render them using the default `html` style.
|
||
|
||
```nix
|
||
{
|
||
outputs = [
|
||
"out"
|
||
"doc"
|
||
];
|
||
|
||
nativeBuildInputs = [
|
||
sphinxHook
|
||
];
|
||
}
|
||
```
|
||
|
||
The hook will automatically build and install the artifact into the
|
||
`doc` output, if it exists. It also provides an automatic diversion
|
||
for the artifacts of the `man` builder into the `man` target.
|
||
|
||
```nix
|
||
{
|
||
outputs = [
|
||
"out"
|
||
"doc"
|
||
"man"
|
||
];
|
||
|
||
# Use multiple builders
|
||
sphinxBuilders = [
|
||
"singlehtml"
|
||
"man"
|
||
];
|
||
}
|
||
```
|
||
|
||
Overwrite `sphinxRoot` when the hook is unable to find your
|
||
documentation source root.
|
||
|
||
```nix
|
||
{
|
||
# Configure sphinxRoot for uncommon paths
|
||
sphinxRoot = "weird/docs/path";
|
||
}
|
||
```
|
||
|
||
The hook is also available to packages outside the python ecosystem by
|
||
referencing it using `sphinxHook` from top-level.
|
||
|
||
### Organising your packages {#organising-your-packages}
|
||
|
||
So far we discussed how you can use Python on Nix, and how you can develop with
|
||
it. We've looked at how you write expressions to package Python packages, and we
|
||
looked at how you can create environments in which specified packages are
|
||
available.
|
||
|
||
At some point you'll likely have multiple packages which you would
|
||
like to be able to use in different projects. In order to minimise unnecessary
|
||
duplication we now look at how you can maintain a repository with your
|
||
own packages. The important functions here are `import` and `callPackage`.
|
||
|
||
### Including a derivation using `callPackage` {#including-a-derivation-using-callpackage}
|
||
|
||
Earlier we created a Python environment using [`withPackages`](#python.withpackages-function), and included the
|
||
`toolz` package via a `let` expression.
|
||
Let's split the package definition from the environment definition.
|
||
|
||
We first create a function that builds `toolz` in `~/path/to/toolz/release.nix`
|
||
|
||
```nix
|
||
{ lib
|
||
, buildPythonPackage
|
||
, fetchPypi
|
||
, setuptools
|
||
}:
|
||
|
||
buildPythonPackage rec {
|
||
pname = "toolz";
|
||
version = "0.10.0";
|
||
pyproject = true;
|
||
|
||
src = fetchPypi {
|
||
inherit pname version;
|
||
hash = "sha256-CP3V73yWSArRHBLUct4hrNMjWZlvaaUlkpm1QP66RWA=";
|
||
};
|
||
|
||
build-system = [
|
||
setuptools
|
||
];
|
||
|
||
meta = {
|
||
changelog = "https://github.com/pytoolz/toolz/releases/tag/${version}";
|
||
homepage = "https://github.com/pytoolz/toolz/";
|
||
description = "List processing tools and functional utilities";
|
||
license = lib.licenses.bsd3;
|
||
};
|
||
}
|
||
```
|
||
|
||
It takes an argument [`buildPythonPackage`](#buildpythonpackage-function). We now call this function using
|
||
`callPackage` in the definition of our environment
|
||
|
||
```nix
|
||
with import <nixpkgs> {};
|
||
|
||
( let
|
||
toolz = callPackage /path/to/toolz/release.nix {
|
||
buildPythonPackage = python3Packages.buildPythonPackage;
|
||
};
|
||
in python3.withPackages (ps: [
|
||
ps.numpy
|
||
toolz
|
||
])
|
||
).env
|
||
```
|
||
|
||
Important to remember is that the Python version for which the package is made
|
||
depends on the `python` derivation that is passed to [`buildPythonPackage`](#buildpythonpackage-function). Nix
|
||
tries to automatically pass arguments when possible, which is why generally you
|
||
don't explicitly define which `python` derivation should be used. In the above
|
||
example we use [`buildPythonPackage`](#buildpythonpackage-function) that is part of the set `python3Packages`,
|
||
and in this case the `python3` interpreter is automatically used.
|
||
|
||
## FAQ {#faq}
|
||
|
||
### How to solve circular dependencies? {#how-to-solve-circular-dependencies}
|
||
|
||
Consider the packages `A` and `B` that depend on each other. When packaging `B`,
|
||
a solution is to override package `A` not to depend on `B` as an input. The same
|
||
should also be done when packaging `A`.
|
||
|
||
### How to override a Python package? {#how-to-override-a-python-package}
|
||
|
||
We can override the interpreter and pass `packageOverrides`. In the following
|
||
example we rename the `pandas` package and build it.
|
||
|
||
```nix
|
||
with import <nixpkgs> {};
|
||
|
||
(let
|
||
python = let
|
||
packageOverrides = self: super: {
|
||
pandas = super.pandas.overridePythonAttrs(old: {name="foo";});
|
||
};
|
||
in pkgs.python310.override {
|
||
inherit packageOverrides;
|
||
};
|
||
|
||
in python.withPackages (ps: [
|
||
ps.pandas
|
||
])).env
|
||
```
|
||
|
||
Using `nix-build` on this expression will build an environment that contains the
|
||
package `pandas` but with the new name `foo`.
|
||
|
||
All packages in the package set will use the renamed package. A typical use case
|
||
is to switch to another version of a certain package. For example, in the
|
||
Nixpkgs repository we have multiple versions of `django` and `scipy`. In the
|
||
following example we use a different version of `scipy` and create an
|
||
environment that uses it. All packages in the Python package set will now use
|
||
the updated `scipy` version.
|
||
|
||
```nix
|
||
with import <nixpkgs> {};
|
||
|
||
( let
|
||
packageOverrides = self: super: {
|
||
scipy = super.scipy_0_17;
|
||
};
|
||
in (pkgs.python310.override {
|
||
inherit packageOverrides;
|
||
}).withPackages (ps: [
|
||
ps.blaze
|
||
])
|
||
).env
|
||
```
|
||
|
||
The requested package `blaze` depends on `pandas` which itself depends on `scipy`.
|
||
|
||
If you want the whole of Nixpkgs to use your modifications, then you can use
|
||
`overlays` as explained in this manual. In the following example we build a
|
||
`inkscape` using a different version of `numpy`.
|
||
|
||
```nix
|
||
let
|
||
pkgs = import <nixpkgs> {};
|
||
newpkgs = import pkgs.path { overlays = [ (self: super: {
|
||
python310 = let
|
||
packageOverrides = python-self: python-super: {
|
||
numpy = python-super.numpy_1_18;
|
||
};
|
||
in super.python310.override {inherit packageOverrides;};
|
||
} ) ]; };
|
||
in newpkgs.inkscape
|
||
```
|
||
|
||
### `python setup.py bdist_wheel` cannot create .whl {#python-setup.py-bdist_wheel-cannot-create-.whl}
|
||
|
||
Executing `python setup.py bdist_wheel` in a `nix-shell`fails with
|
||
|
||
```
|
||
ValueError: ZIP does not support timestamps before 1980
|
||
```
|
||
|
||
This is because files from the Nix store (which have a timestamp of the UNIX
|
||
epoch of January 1, 1970) are included in the .ZIP, but .ZIP archives follow the
|
||
DOS convention of counting timestamps from 1980.
|
||
|
||
The command `bdist_wheel` reads the `SOURCE_DATE_EPOCH` environment variable,
|
||
which `nix-shell` sets to 1. Unsetting this variable or giving it a value
|
||
corresponding to 1980 or later enables building wheels.
|
||
|
||
Use 1980 as timestamp:
|
||
|
||
```shell
|
||
nix-shell --run "SOURCE_DATE_EPOCH=315532800 python3 setup.py bdist_wheel"
|
||
```
|
||
|
||
or the current time:
|
||
|
||
```shell
|
||
nix-shell --run "SOURCE_DATE_EPOCH=$(date +%s) python3 setup.py bdist_wheel"
|
||
```
|
||
|
||
or unset `SOURCE_DATE_EPOCH`:
|
||
|
||
```shell
|
||
nix-shell --run "unset SOURCE_DATE_EPOCH; python3 setup.py bdist_wheel"
|
||
```
|
||
|
||
### `install_data` / `data_files` problems {#install_data-data_files-problems}
|
||
|
||
If you get the following error:
|
||
|
||
```
|
||
could not create '/nix/store/6l1bvljpy8gazlsw2aw9skwwp4pmvyxw-python-2.7.8/etc':
|
||
Permission denied
|
||
```
|
||
|
||
This is a [known bug](https://github.com/pypa/setuptools/issues/130) in
|
||
`setuptools`. Setuptools `install_data` does not respect `--prefix`. An example
|
||
of such package using the feature is `pkgs/tools/X11/xpra/default.nix`.
|
||
|
||
As workaround install it as an extra `preInstall` step:
|
||
|
||
```shell
|
||
${python.pythonOnBuildForHost.interpreter} setup.py install_data --install-dir=$out --root=$out
|
||
sed -i '/ = data\_files/d' setup.py
|
||
```
|
||
|
||
### Rationale of non-existent global site-packages {#rationale-of-non-existent-global-site-packages}
|
||
|
||
On most operating systems a global `site-packages` is maintained. This however
|
||
becomes problematic if you want to run multiple Python versions or have multiple
|
||
versions of certain libraries for your projects. Generally, you would solve such
|
||
issues by creating virtual environments using `virtualenv`.
|
||
|
||
On Nix each package has an isolated dependency tree which, in the case of
|
||
Python, guarantees the right versions of the interpreter and libraries or
|
||
packages are available. There is therefore no need to maintain a global `site-packages`.
|
||
|
||
If you want to create a Python environment for development, then the recommended
|
||
method is to use `nix-shell`, either with or without the [`python.buildEnv`](#python.buildenv-function)
|
||
function.
|
||
|
||
### How to consume Python modules using pip in a virtual environment like I am used to on other Operating Systems? {#how-to-consume-python-modules-using-pip-in-a-virtual-environment-like-i-am-used-to-on-other-operating-systems}
|
||
|
||
While this approach is not very idiomatic from Nix perspective, it can still be
|
||
useful when dealing with pre-existing projects or in situations where it's not
|
||
feasible or desired to write derivations for all required dependencies.
|
||
|
||
This is an example of a `default.nix` for a `nix-shell`, which allows to consume
|
||
a virtual environment created by `venv`, and install Python modules through
|
||
`pip` the traditional way.
|
||
|
||
Create this `default.nix` file, together with a `requirements.txt` and
|
||
execute `nix-shell`.
|
||
|
||
```nix
|
||
with import <nixpkgs> { };
|
||
|
||
let
|
||
pythonPackages = python3Packages;
|
||
in pkgs.mkShell rec {
|
||
name = "impurePythonEnv";
|
||
venvDir = "./.venv";
|
||
buildInputs = [
|
||
# A Python interpreter including the 'venv' module is required to bootstrap
|
||
# the environment.
|
||
pythonPackages.python
|
||
|
||
# This executes some shell code to initialize a venv in $venvDir before
|
||
# dropping into the shell
|
||
pythonPackages.venvShellHook
|
||
|
||
# Those are dependencies that we would like to use from nixpkgs, which will
|
||
# add them to PYTHONPATH and thus make them accessible from within the venv.
|
||
pythonPackages.numpy
|
||
pythonPackages.requests
|
||
|
||
# In this particular example, in order to compile any binary extensions they may
|
||
# require, the Python modules listed in the hypothetical requirements.txt need
|
||
# the following packages to be installed locally:
|
||
taglib
|
||
openssl
|
||
git
|
||
libxml2
|
||
libxslt
|
||
libzip
|
||
zlib
|
||
];
|
||
|
||
# Run this command, only after creating the virtual environment
|
||
postVenvCreation = ''
|
||
unset SOURCE_DATE_EPOCH
|
||
pip install -r requirements.txt
|
||
'';
|
||
|
||
# Now we can execute any commands within the virtual environment.
|
||
# This is optional and can be left out to run pip manually.
|
||
postShellHook = ''
|
||
# allow pip to install wheels
|
||
unset SOURCE_DATE_EPOCH
|
||
'';
|
||
|
||
}
|
||
```
|
||
|
||
In case the supplied venvShellHook is insufficient, or when Python 2 support is
|
||
needed, you can define your own shell hook and adapt to your needs like in the
|
||
following example:
|
||
|
||
```nix
|
||
with import <nixpkgs> { };
|
||
|
||
let
|
||
venvDir = "./.venv";
|
||
pythonPackages = python3Packages;
|
||
in pkgs.mkShell rec {
|
||
name = "impurePythonEnv";
|
||
buildInputs = [
|
||
pythonPackages.python
|
||
# Needed when using python 2.7
|
||
# pythonPackages.virtualenv
|
||
# ...
|
||
];
|
||
|
||
# This is very close to how venvShellHook is implemented, but
|
||
# adapted to use 'virtualenv'
|
||
shellHook = ''
|
||
SOURCE_DATE_EPOCH=$(date +%s)
|
||
|
||
if [ -d "${venvDir}" ]; then
|
||
echo "Skipping venv creation, '${venvDir}' already exists"
|
||
else
|
||
echo "Creating new venv environment in path: '${venvDir}'"
|
||
# Note that the module venv was only introduced in python 3, so for 2.7
|
||
# this needs to be replaced with a call to virtualenv
|
||
${pythonPackages.python.interpreter} -m venv "${venvDir}"
|
||
fi
|
||
|
||
# Under some circumstances it might be necessary to add your virtual
|
||
# environment to PYTHONPATH, which you can do here too;
|
||
# PYTHONPATH=$PWD/${venvDir}/${pythonPackages.python.sitePackages}/:$PYTHONPATH
|
||
|
||
source "${venvDir}/bin/activate"
|
||
|
||
# As in the previous example, this is optional.
|
||
pip install -r requirements.txt
|
||
'';
|
||
}
|
||
```
|
||
|
||
Note that the `pip install` is an imperative action. So every time `nix-shell`
|
||
is executed it will attempt to download the Python modules listed in
|
||
requirements.txt. However these will be cached locally within the `virtualenv`
|
||
folder and not downloaded again.
|
||
|
||
### How to override a Python package from `configuration.nix`? {#how-to-override-a-python-package-from-configuration.nix}
|
||
|
||
If you need to change a package's attribute(s) from `configuration.nix` you could do:
|
||
|
||
```nix
|
||
{
|
||
nixpkgs.config.packageOverrides = super: {
|
||
python3 = super.python3.override {
|
||
packageOverrides = python-self: python-super: {
|
||
twisted = python-super.twisted.overridePythonAttrs (oldAttrs: {
|
||
src = super.fetchPypi {
|
||
pname = "Twisted";
|
||
version = "19.10.0";
|
||
hash = "sha256-c5S6fycq5yKnTz2Wnc9Zm8TvCTvDkgOHSKSQ8XJKUV0=";
|
||
extension = "tar.bz2";
|
||
};
|
||
});
|
||
};
|
||
};
|
||
};
|
||
}
|
||
```
|
||
|
||
`python3Packages.twisted` is now globally overridden.
|
||
All packages and also all NixOS services that reference `twisted`
|
||
(such as `services.buildbot-worker`) now use the new definition.
|
||
Note that `python-super` refers to the old package set and `python-self`
|
||
to the new, overridden version.
|
||
|
||
To modify only a Python package set instead of a whole Python derivation, use
|
||
this snippet:
|
||
|
||
```nix
|
||
{
|
||
myPythonPackages = python3Packages.override {
|
||
overrides = self: super: {
|
||
twisted = <...>;
|
||
};
|
||
};
|
||
}
|
||
```
|
||
|
||
### How to override a Python package using overlays? {#how-to-override-a-python-package-using-overlays}
|
||
|
||
Use the following overlay template:
|
||
|
||
```nix
|
||
self: super: {
|
||
python = super.python.override {
|
||
packageOverrides = python-self: python-super: {
|
||
twisted = python-super.twisted.overrideAttrs (oldAttrs: {
|
||
src = super.fetchPypi {
|
||
pname = "Twisted";
|
||
version = "19.10.0";
|
||
hash = "sha256-c5S6fycq5yKnTz2Wnc9Zm8TvCTvDkgOHSKSQ8XJKUV0=";
|
||
extension = "tar.bz2";
|
||
};
|
||
});
|
||
};
|
||
};
|
||
}
|
||
```
|
||
|
||
### How to override a Python package for all Python versions using extensions? {#how-to-override-a-python-package-for-all-python-versions-using-extensions}
|
||
|
||
The following overlay overrides the call to [`buildPythonPackage`](#buildpythonpackage-function) for the
|
||
`foo` package for all interpreters by appending a Python extension to the
|
||
`pythonPackagesExtensions` list of extensions.
|
||
|
||
```nix
|
||
final: prev: {
|
||
pythonPackagesExtensions = prev.pythonPackagesExtensions ++ [
|
||
(
|
||
python-final: python-prev: {
|
||
foo = python-prev.foo.overridePythonAttrs (oldAttrs: {
|
||
# ...
|
||
});
|
||
}
|
||
)
|
||
];
|
||
}
|
||
```
|
||
|
||
### How to use Intel’s MKL with numpy and scipy? {#how-to-use-intels-mkl-with-numpy-and-scipy}
|
||
|
||
MKL can be configured using an overlay. See the section "[Using overlays to
|
||
configure alternatives](#sec-overlays-alternatives-blas-lapack)".
|
||
|
||
### What inputs do `setup_requires`, `install_requires` and `tests_require` map to? {#what-inputs-do-setup_requires-install_requires-and-tests_require-map-to}
|
||
|
||
In a `setup.py` or `setup.cfg` it is common to declare dependencies:
|
||
|
||
* `setup_requires` corresponds to `build-system`
|
||
* `install_requires` corresponds to `dependencies`
|
||
* `tests_require` corresponds to [`nativeCheckInputs`](#var-stdenv-nativeCheckInputs)
|
||
|
||
### How to enable interpreter optimizations? {#optimizations}
|
||
|
||
The Python interpreters are by default not built with optimizations enabled, because
|
||
the builds are in that case not reproducible. To enable optimizations, override the
|
||
interpreter of interest, e.g using
|
||
|
||
```nix
|
||
let
|
||
pkgs = import ./. {};
|
||
mypython = pkgs.python3.override {
|
||
enableOptimizations = true;
|
||
reproducibleBuild = false;
|
||
self = mypython;
|
||
};
|
||
in mypython
|
||
```
|
||
|
||
### How to add optional dependencies? {#python-optional-dependencies}
|
||
|
||
Some packages define optional dependencies for additional features. With
|
||
`setuptools` this is called `extras_require` and `flit` calls it
|
||
`extras-require`, while PEP 621 calls these `optional-dependencies`.
|
||
|
||
```nix
|
||
{
|
||
optional-dependencies = {
|
||
complete = [ distributed ];
|
||
};
|
||
}
|
||
```
|
||
|
||
and letting the package requiring the extra add the list to its dependencies
|
||
|
||
```nix
|
||
{
|
||
dependencies = [
|
||
# ...
|
||
] ++ dask.optional-dependencies.complete;
|
||
}
|
||
```
|
||
|
||
This method is using `passthru`, meaning that changing `optional-dependencies` of a package won't cause it to rebuild.
|
||
|
||
Note this method is preferred over adding parameters to builders, as that can
|
||
result in packages depending on different variants and thereby causing
|
||
collisions.
|
||
|
||
### How to contribute a Python package to nixpkgs? {#tools}
|
||
|
||
Packages inside nixpkgs must use the [`buildPythonPackage`](#buildpythonpackage-function) or [`buildPythonApplication`](#buildpythonapplication-function) function directly,
|
||
because we can only provide security support for non-vendored dependencies.
|
||
|
||
We recommend [nix-init](https://github.com/nix-community/nix-init) for creating new python packages within nixpkgs,
|
||
as it already prefetches the source, parses dependencies for common formats and prefills most things in `meta`.
|
||
|
||
See also [contributing section](#contributing).
|
||
|
||
### Are Python interpreters built deterministically? {#deterministic-builds}
|
||
|
||
The Python interpreters are now built deterministically. Minor modifications had
|
||
to be made to the interpreters in order to generate deterministic bytecode. This
|
||
has security implications and is relevant for those using Python in a
|
||
`nix-shell`.
|
||
|
||
When the environment variable `DETERMINISTIC_BUILD` is set, all bytecode will
|
||
have timestamp 1. The [`buildPythonPackage`](#buildpythonpackage-function) function sets `DETERMINISTIC_BUILD=1`
|
||
and [PYTHONHASHSEED=0](https://docs.python.org/3.12/using/cmdline.html#envvar-PYTHONHASHSEED).
|
||
Both are also exported in `nix-shell`.
|
||
|
||
### How to provide automatic tests to Python packages? {#automatic-tests}
|
||
|
||
It is recommended to test packages as part of the build process.
|
||
Source distributions (`sdist`) often include test files, but not always.
|
||
|
||
The best practice today is to pass a test hook (e.g. pytestCheckHook, unittestCheckHook) into nativeCheckInputs.
|
||
This will reconfigure the checkPhase to make use of that particular test framework.
|
||
Occasionally packages don't make use of a common test framework, which may then require a custom checkPhase.
|
||
|
||
#### Common issues {#common-issues}
|
||
|
||
* Non-working tests can often be deselected. Most Python modules
|
||
do follow the standard test protocol where the pytest runner can be used.
|
||
`pytest` supports the `-k` and `--ignore` parameters to ignore test
|
||
methods or classes as well as whole files. For `pytestCheckHook` these are
|
||
conveniently exposed as `disabledTests` and `disabledTestPaths` respectively.
|
||
|
||
```nix
|
||
buildPythonPackage {
|
||
# ...
|
||
nativeCheckInputs = [
|
||
pytestCheckHook
|
||
];
|
||
|
||
disabledTests = [
|
||
"function_name"
|
||
"other_function"
|
||
];
|
||
|
||
disabledTestPaths = [
|
||
"this/file.py"
|
||
];
|
||
}
|
||
```
|
||
|
||
* Tests that attempt to access `$HOME` can be fixed by using the following
|
||
work-around before running tests (e.g. `preCheck`): `export HOME=$(mktemp -d)`
|
||
* Compiling with Cython causes tests to fail with a `ModuleNotLoadedError`.
|
||
This can be fixed with two changes in the derivation: 1) replacing `pytest` with
|
||
`pytestCheckHook` and 2) adding a `preCheck` containing `cd $out` to run
|
||
tests within the built output.
|
||
|
||
## Contributing {#contributing}
|
||
|
||
### Contributing guidelines {#contributing-guidelines}
|
||
|
||
The following rules are desired to be respected:
|
||
|
||
* Python libraries are called from `python-packages.nix` and packaged with
|
||
[`buildPythonPackage`](#buildpythonpackage-function). The expression of a library should be in
|
||
`pkgs/development/python-modules/<name>/default.nix`.
|
||
* Python applications live outside of `python-packages.nix` and are packaged
|
||
with [`buildPythonApplication`](#buildpythonapplication-function).
|
||
* Make sure libraries build for all Python interpreters.
|
||
If it fails to build on some Python versions, consider disabling them by setting `disable = pythonAtLeast "3.x"` along with a comment.
|
||
* The two parameters, `pyproject` and `build-system` are set to avoid the legacy setuptools/distutils build.
|
||
* Only unversioned attributes (e.g. `pydantic`, but not `pypdantic_1`) can be included in `dependencies`,
|
||
since due to `PYTHONPATH` limitations we can only ever support a single version for libraries
|
||
without running into duplicate module name conflicts.
|
||
* The version restrictions of `dependencies` can be relaxed by [`pythonRelaxDepsHook`](#using-pythonrelaxdepshook).
|
||
* Make sure the tests are enabled using for example [`pytestCheckHook`](#using-pytestcheckhook) and, in the case of
|
||
libraries, are passing for all interpreters. If certain tests fail they can be
|
||
disabled individually. Try to avoid disabling the tests altogether. In any
|
||
case, when you disable tests, leave a comment explaining why.
|
||
* `pythonImportsCheck` is set. This is still a good smoke test even if `pytestCheckHook` is set.
|
||
* `meta.platforms` takes the default value in many cases.
|
||
It does not need to be set explicitly unless the package requires a specific platform.
|
||
* The file is formatted with `nixfmt-rfc-style`.
|
||
* Commit names of Python libraries must reflect that they are Python
|
||
libraries (e.g. `python312Packages.numpy: 1.11 -> 1.12` rather than `numpy: 1.11 -> 1.12`).
|
||
* The current default version of python should be included
|
||
in commit messages to enable automatic builds by ofborg.
|
||
For example `python312Packages.numpy: 1.11 -> 1.12` should be used rather
|
||
than `python3Packages.numpy: 1.11 -> 1.12`.
|
||
Note that `pythonPackages` is an alias for `python27Packages`.
|
||
* Attribute names in `python-packages.nix` as well as `pname`s should match the
|
||
library's name on PyPI, but be normalized according to [PEP
|
||
0503](https://www.python.org/dev/peps/pep-0503/#normalized-names). This means
|
||
that characters should be converted to lowercase and `.` and `_` should be
|
||
replaced by a single `-` (foo-bar-baz instead of Foo__Bar.baz).
|
||
If necessary, `pname` has to be given a different value within `fetchPypi`.
|
||
* Packages from sources such as GitHub and GitLab that do not exist on PyPI
|
||
should not use a name that is already used on PyPI. When possible, they should
|
||
use the package repository name prefixed with the owner (e.g. organization) name
|
||
and using a `-` as delimiter.
|
||
* Attribute names in `python-packages.nix` should be sorted alphanumerically to
|
||
avoid merge conflicts and ease locating attributes.
|
||
|
||
This list is useful for reviewers as well as for self-checking when submitting packages.
|
||
|
||
## Package set maintenance {#python-package-set-maintenance}
|
||
|
||
The whole Python package set has a lot of packages that do not see regular
|
||
updates, because they either are a very fragile component in the Python
|
||
ecosystem, like for example the `hypothesis` package, or packages that have
|
||
no maintainer, so maintenance falls back to the package set maintainers.
|
||
|
||
### Updating packages in bulk {#python-package-bulk-updates}
|
||
|
||
A tool to bulk-update numerous Python libraries is available in the
|
||
repository at `maintainers/scripts/update-python-libraries`.
|
||
|
||
It can quickly update minor or major versions for all packages selected
|
||
and create update commits, and supports the `fetchPypi`, `fetchurl` and
|
||
`fetchFromGitHub` fetchers. When updating lots of packages that are
|
||
hosted on GitHub, exporting a `GITHUB_API_TOKEN` is highly recommended.
|
||
|
||
Updating packages in bulk leads to lots of breakages, which is why a
|
||
stabilization period on the `python-updates` branch is required.
|
||
|
||
If a package is fragile and often breaks during these bulks updates, it
|
||
may be reasonable to set `passthru.skipBulkUpdate = true` in the
|
||
derivation. This decision should not be made on a whim and should
|
||
always be supported by a qualifying comment.
|
||
|
||
Once the branch is sufficiently stable it should normally be merged
|
||
into the `staging` branch.
|
||
|
||
An exemplary call to update all python libraries between minor versions
|
||
would be:
|
||
|
||
```ShellSession
|
||
$ maintainers/scripts/update-python-libraries --target minor --commit --use-pkgs-prefix pkgs/development/python-modules/**/default.nix
|
||
```
|
||
|
||
## CPython Update Schedule {#python-cpython-update-schedule}
|
||
|
||
With [PEP 602](https://www.python.org/dev/peps/pep-0602/), CPython now
|
||
follows a yearly release cadence. In nixpkgs, all supported interpreters
|
||
are made available, but only the most recent two
|
||
interpreters package sets are built; this is a compromise between being
|
||
the latest interpreter, and what the majority of the Python packages support.
|
||
|
||
New CPython interpreters are released in October. Generally, it takes some
|
||
time for the majority of active Python projects to support the latest stable
|
||
interpreter. To help ease the migration for Nixpkgs users
|
||
between Python interpreters the schedule below will be used:
|
||
|
||
| When | Event |
|
||
| --- | --- |
|
||
| After YY.11 Release | Bump CPython package set window. The latest and previous latest stable should now be built. |
|
||
| After YY.05 Release | Bump default CPython interpreter to latest stable. |
|
||
|
||
In practice, this means that the Python community will have had a stable interpreter
|
||
for ~2 months before attempting to update the package set. And this will
|
||
allow for ~7 months for Python applications to support the latest interpreter.
|