pytest import mechanisms and sys.path/PYTHONPATH

Import modes

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

Importing files in Python is a non-trivial process, so 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 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.

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

    This better allows users 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-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: this mode uses more fine control mechanisms provided by importlib to import test modules, without changing sys.path.

    Advantages of this mode:

    • pytest will not change 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 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 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 sys.path.

      Because Python requires the module to also be available in sys.modules, pytest derives a unique name for it based on its relative location from the rootdir, and adds the module to sys.modules.

      For example, tests/core/test_models.py will end up being imported as the module tests.core.test_models.

    Added in version 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.

Note

By default, pytest will not attempt to resolve namespace packages automatically, but that can be changed via the consider_namespace_packages configuration variable.

See also

The pythonpath configuration variable.

The consider_namespace_packages configuration variable.

Choosing a 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 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 Conventions for Python test discovery.

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 Conventions for Python test discovery.

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 is standard python behavior.

See also Calling pytest through python -m pytest.