doc: update and improve import mode docs

doc/en/explanation/goodpractices.rst

@@ -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:
 

doc/en/explanation/pythonpath.rst

@@ -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*
+.. _`import-mode-prepend`:
-  of :py:data:`sys.path` if not already there, and then imported with the :func:`importlib.import_module <importlib.import_module>` function.
 
-  This requires test module names to be unique when the test directory tree is not arranged in
+* ``prepend`` (default): the directory path containing each module will be inserted into the *beginning*
-  packages, because the modules will put in :py:data:`sys.modules` after importing.
+  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>`.
 
@@ -38,18 +47,55 @@ 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
+  Advantages of this mode:
-  modules in the tests directories are not automatically importable because the tests directory is no longer
+
-  added to :py:data:`sys.path`.
+  * 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.
@@ -58,12 +104,14 @@ these values:
 
     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
+: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 ``sys.path`` to import it as ``conftest``.
+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`.