doc: update and improve import mode docs

This commit is contained in:
Bruno Oliveira 2024-02-27 21:29:59 +02:00
parent c85fce39b6
commit 5746b8e696
2 changed files with 69 additions and 19 deletions

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@ -60,8 +60,10 @@ Within Python modules, ``pytest`` also discovers tests using the standard
:ref:`unittest.TestCase <unittest.TestCase>` subclassing technique.
Choosing a test layout / import rules
-------------------------------------
.. _`test layout`:
Choosing a test layout
----------------------
``pytest`` supports two common test layouts:

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@ -15,14 +15,23 @@ changing :data:`sys.path`. Some aspects of the
import process can be controlled through the ``--import-mode`` command-line flag, which can assume
these values:
* ``prepend`` (default): the directory path containing each module will be inserted into the *beginning*
of :py:data:`sys.path` if not already there, and then imported with the :func:`importlib.import_module <importlib.import_module>` function.
.. _`import-mode-prepend`:
This requires test module names to be unique when the test directory tree is not arranged in
packages, because the modules will put in :py:data:`sys.modules` after importing.
* ``prepend`` (default): the directory path containing each module will be inserted into the *beginning*
of :py:data:`sys.path` if not already there, and then imported with
the :func:`importlib.import_module <importlib.import_module>` function.
It is highly recommended to arrange your test modules as packages by adding ``__init__.py`` files to your directories
containing tests. This will make the tests part of a proper Python package, allowing pytest to resolve their full
name (for example ``tests.core.test_core`` for ``test_core.py`` inside the ``tests.core`` package).
If the test directory tree is not arranged as packages, then each test file needs to have a unique name
compared to the other test files, otherwise pytest will raise an error if it finds two tests with the same name.
This is the classic mechanism, dating back from the time Python 2 was still supported.
.. _`import-mode-append`:
* ``append``: the directory containing each module is appended to the end of :py:data:`sys.path` if not already
there, and imported with :func:`importlib.import_module <importlib.import_module>`.
@ -38,32 +47,71 @@ these values:
the tests will run against the installed version
of ``pkg_under_test`` when ``--import-mode=append`` is used whereas
with ``prepend`` they would pick up the local version. This kind of confusion is why
we advocate for using :ref:`src <src-layout>` layouts.
we advocate for using :ref:`src-layouts <src-layout>`.
Same as ``prepend``, requires test module names to be unique when the test directory tree is
not arranged in packages, because the modules will put in :py:data:`sys.modules` after importing.
* ``importlib``: new in pytest-6.0, this mode uses more fine control mechanisms provided by :mod:`importlib` to import test modules. This gives full control over the import process, and doesn't require changing :py:data:`sys.path`.
.. _`import-mode-importlib`:
For this reason this doesn't require test module names to be unique.
* ``importlib``: this mode uses more fine control mechanisms provided by :mod:`importlib` to import test modules, without changing :py:data:`sys.path`.
One drawback however is that test modules are non-importable by each other. Also, utility
modules in the tests directories are not automatically importable because the tests directory is no longer
added to :py:data:`sys.path`.
Advantages of this mode:
Initially we intended to make ``importlib`` the default in future releases, however it is clear now that
it has its own set of drawbacks so the default will remain ``prepend`` for the foreseeable future.
* pytest will not change :py:data:`sys.path` at all.
* Test module names do not need to be unique -- pytest will generate a unique name automatically based on the ``rootdir``.
Disadvantages:
* Test modules can't import each other.
* Testing utility modules in the tests directories (for example a ``tests.helpers`` module containing test-related functions/classes)
are not importable. The recommendation in this case it to place testing utility modules together with the application/library
code, for example ``app.testing.helpers``.
Important: by "test utility modules" we mean functions/classes which are imported by
other tests directly; this does not include fixtures, which should be placed in ``conftest.py`` files, along
with the test modules, and are discovered automatically by pytest.
It works like this:
1. Given a certain module path, for example ``tests/core/test_models.py``, derives a canonical name
like ``tests.core.test_models`` and tries to import it.
For non-test modules this will work if they are accessible via :py:data:`sys.path`, so
for example ``.env/lib/site-packages/app/core.py`` will be importable as ``app.core``.
This is happens when plugins import non-test modules (for example doctesting).
If this step succeeds, the module is returned.
For test modules, unless they are reachable from :py:data:`sys.path`, this step will fail.
2. If the previous step fails, we import the module directly using ``importlib`` facilities, which lets us import it without
changing :py:data:`sys.path`.
Because Python requires the module to also be available in :py:data:`sys.modules`, pytest derives a unique name for it based
on its relative location from the ``rootdir``, and adds the module to :py:data:`sys.modules`.
For example, ``tests/core/test_models.py`` will end up being imported as the module ``tests.core.test_models``.
.. versionadded:: 6.0
.. note::
Initially we intended to make ``importlib`` the default in future releases, however it is clear now that
it has its own set of drawbacks so the default will remain ``prepend`` for the foreseeable future.
.. seealso::
The :confval:`pythonpath` configuration variable.
:ref:`test layout`.
``prepend`` and ``append`` import modes scenarios
-------------------------------------------------
Here's a list of scenarios when using ``prepend`` or ``append`` import modes where pytest needs to
change ``sys.path`` in order to import test modules or ``conftest.py`` files, and the issues users
change :py:data:`sys.path` in order to import test modules or ``conftest.py`` files, and the issues users
might encounter because of that.
Test modules / ``conftest.py`` files inside packages
@ -92,7 +140,7 @@ pytest will find ``foo/bar/tests/test_foo.py`` and realize it is part of a packa
there's an ``__init__.py`` file in the same folder. It will then search upwards until it can find the
last folder which still contains an ``__init__.py`` file in order to find the package *root* (in
this case ``foo/``). To load the module, it will insert ``root/`` to the front of
``sys.path`` (if not there already) in order to load
:py:data:`sys.path` (if not there already) in order to load
``test_foo.py`` as the *module* ``foo.bar.tests.test_foo``.
The same logic applies to the ``conftest.py`` file: it will be imported as ``foo.conftest`` module.
@ -122,8 +170,8 @@ When executing:
pytest will find ``foo/bar/tests/test_foo.py`` and realize it is NOT part of a package given that
there's no ``__init__.py`` file in the same folder. It will then add ``root/foo/bar/tests`` to
``sys.path`` in order to import ``test_foo.py`` as the *module* ``test_foo``. The same is done
with the ``conftest.py`` file by adding ``root/foo`` to ``sys.path`` to import it as ``conftest``.
:py:data:`sys.path` in order to import ``test_foo.py`` as the *module* ``test_foo``. The same is done
with the ``conftest.py`` file by adding ``root/foo`` to :py:data:`sys.path` to import it as ``conftest``.
For this reason this layout cannot have test modules with the same name, as they all will be
imported in the global import namespace.
@ -136,7 +184,7 @@ Invoking ``pytest`` versus ``python -m pytest``
-----------------------------------------------
Running pytest with ``pytest [...]`` instead of ``python -m pytest [...]`` yields nearly
equivalent behaviour, except that the latter will add the current directory to ``sys.path``, which
equivalent behaviour, except that the latter will add the current directory to :py:data:`sys.path`, which
is standard ``python`` behavior.
See also :ref:`invoke-python`.