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 ofsys.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 ofsys.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 withprepend
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 insys.modules
after importing.importlib
: new in pytest-6.0, this mode usesimportlib
to import test modules. This gives full control over the import process, and doesn’t require changingsys.path
.For this reason this doesn’t require test module names to be unique, but also makes test modules non-importable by each other.
We intend to make
importlib
the default in future releases, depending on feedback.
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.