pytest import mechanisms and sys.path/PYTHONPATH

Here’s a list of scenarios where pytest may need to change sys.path in order to import test modules or conftest.py files.

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.