143 lines
5.3 KiB
ReStructuredText
143 lines
5.3 KiB
ReStructuredText
.. _pythonpath:
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pytest import mechanisms and ``sys.path``/``PYTHONPATH``
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========================================================
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.. _`import-modes`:
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Import modes
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------------
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pytest as a testing framework needs to import test modules and ``conftest.py`` files for execution.
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Importing files in Python (at least until recently) is a non-trivial processes, often requiring
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changing :data:`sys.path`. Some aspects of the
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import process can be controlled through the ``--import-mode`` command-line flag, which can assume
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these values:
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* ``prepend`` (default): the directory path containing each module will be inserted into the *beginning*
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of :py:data:`sys.path` if not already there, and then imported with the :func:`__import__ <__import__>` builtin.
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This requires test module names to be unique when the test directory tree is not arranged in
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packages, because the modules will put in :py:data:`sys.modules` after importing.
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This is the classic mechanism, dating back from the time Python 2 was still supported.
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* ``append``: the directory containing each module is appended to the end of :py:data:`sys.path` if not already
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there, and imported with ``__import__``.
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This better allows to run test modules against installed versions of a package even if the
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package under test has the same import root. For example:
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::
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testing/__init__.py
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testing/test_pkg_under_test.py
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pkg_under_test/
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the tests will run against the installed version
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of ``pkg_under_test`` when ``--import-mode=append`` is used whereas
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with ``prepend`` they would pick up the local version. This kind of confusion is why
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we advocate for using :ref:`src <src-layout>` layouts.
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Same as ``prepend``, requires test module names to be unique when the test directory tree is
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not arranged in packages, because the modules will put in :py:data:`sys.modules` after importing.
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* ``importlib``: new in pytest-6.0, this mode uses :mod:`importlib` to import test modules. This gives full control over the import process, and doesn't require changing :py:data:`sys.path`.
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For this reason this doesn't require test module names to be unique.
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One drawback however is that test modules are non-importable by each other. Also, utility
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modules in the tests directories are not automatically importable because the tests directory is no longer
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added to :py:data:`sys.path`.
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Initially we intended to make ``importlib`` the default in future releases, however it is clear now that
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it has its own set of drawbacks so the default will remain ``prepend`` for the foreseeable future.
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.. seealso::
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The :confval:`pythonpath` configuration variable.
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``prepend`` and ``append`` import modes scenarios
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-------------------------------------------------
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Here's a list of scenarios when using ``prepend`` or ``append`` import modes where pytest needs to
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change ``sys.path`` in order to import test modules or ``conftest.py`` files, and the issues users
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might encounter because of that.
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Test modules / ``conftest.py`` files inside packages
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Consider this file and directory layout::
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root/
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|- foo/
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|- __init__.py
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|- conftest.py
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|- bar/
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|- __init__.py
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|- tests/
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|- __init__.py
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|- test_foo.py
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When executing:
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.. code-block:: bash
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pytest root/
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pytest will find ``foo/bar/tests/test_foo.py`` and realize it is part of a package given that
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there's an ``__init__.py`` file in the same folder. It will then search upwards until it can find the
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last folder which still contains an ``__init__.py`` file in order to find the package *root* (in
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this case ``foo/``). To load the module, it will insert ``root/`` to the front of
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``sys.path`` (if not there already) in order to load
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``test_foo.py`` as the *module* ``foo.bar.tests.test_foo``.
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The same logic applies to the ``conftest.py`` file: it will be imported as ``foo.conftest`` module.
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Preserving the full package name is important when tests live in a package to avoid problems
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and allow test modules to have duplicated names. This is also discussed in details in
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:ref:`test discovery`.
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Standalone test modules / ``conftest.py`` files
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Consider this file and directory layout::
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root/
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|- foo/
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|- conftest.py
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|- bar/
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|- tests/
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|- test_foo.py
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When executing:
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.. code-block:: bash
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pytest root/
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pytest will find ``foo/bar/tests/test_foo.py`` and realize it is NOT part of a package given that
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there's no ``__init__.py`` file in the same folder. It will then add ``root/foo/bar/tests`` to
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``sys.path`` in order to import ``test_foo.py`` as the *module* ``test_foo``. The same is done
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with the ``conftest.py`` file by adding ``root/foo`` to ``sys.path`` to import it as ``conftest``.
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For this reason this layout cannot have test modules with the same name, as they all will be
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imported in the global import namespace.
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This is also discussed in details in :ref:`test discovery`.
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.. _`pytest vs python -m pytest`:
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Invoking ``pytest`` versus ``python -m pytest``
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-----------------------------------------------
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Running pytest with ``pytest [...]`` instead of ``python -m pytest [...]`` yields nearly
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equivalent behaviour, except that the latter will add the current directory to ``sys.path``, which
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is standard ``python`` behavior.
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See also :ref:`invoke-python`.
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