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 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.

  For this reason this doesn’t require test module names to be unique.

  One drawback however is that test modules are non-importable by each other. Also, utility modules in the tests directories are not automatically importable because the tests directory is no longer added to sys.path.

  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.

参见

The pythonpath configuration variable.

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 通过 python -m pytest 调用 pytest.