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# 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 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 be 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 is 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 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 directory. It will then search upwards until it can find the last directory 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 detail 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 directory. 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 detail 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.