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Pytest-pytest import mechanisms and `sys.path`/`PYTHONPATH`

Import modes

pytest as a testing framework needs to import test modules and
conftest.py files for execution.

Importing files in Python (at least until recently) is a non-trivial
processes, often requiring changing
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 sys.path if not already there,
    and then imported with the
    __import__
    builtin.

    This requires test module names to be unique when the test directory
    tree is not arranged in packages, because the modules will put in
    sys.modules after importing.

    This is the classic mechanism, dating back from the time Python 2
    was still supported.

  • append: the directory containing each module is appended to the
    end of sys.path if not already there, and imported with
    __import__.

    This better allows to run test modules against installed versions of
    a package even if the package under test has the same import root.
    For example:

    testing/__init__.py
    testing/test_pkg_under_test.py
    pkg_under_test/
    

    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 src <src-layout>{.interpreted-text role=“ref”}
    layouts.

    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 sys.modules after importing.

  • importlib: new in pytest-6.0, this mode uses
    importlib to
    import test modules. This gives full control over the import
    process, and doesn’t require changing sys.path or sys.modules
    at all.

    For this reason this doesn’t require test module names to be unique
    at all, but also makes test modules non-importable by each other.
    This was made possible in previous modes, for tests not residing in
    Python packages, because of the side-effects of changing sys.path
    and sys.modules mentioned above. Users which require this should
    turn their tests into proper packages instead.

    We intend to make importlib the default in future releases.

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 might encounter
because of that.

Test modules / conftest.py files inside packages

Consider this file and directory layout:

root/
|- foo/
   |- __init__.py
   |- conftest.py
   |- bar/
      |- __init__.py
      |- tests/
         |- __init__.py
         |- test_foo.py

When executing:

pytest root/

pytest will find foo/bar/tests/test_foo.py and realize it is part of a
package given that 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
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.

Preserving the full package name is important when tests live in a
package to avoid problems and allow test modules to have duplicated
names. This is also discussed in details in
test discovery{.interpreted-text role=“ref”}.

Standalone test modules / conftest.py files

Consider this file and directory layout:

root/
|- foo/
   |- conftest.py
   |- bar/
      |- tests/
         |- test_foo.py

When executing:

pytest root/

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.

For this reason this layout cannot have test modules with the same name,
as they all will be imported in the global import namespace.

This is also discussed in details in test discovery{.interpreted-text
role=“ref”}.

Invoking pytest versus python -m pytest {#pytest vs 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 is standard python behavior.

See also cmdline{.interpreted-text role=“ref”}.