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Pytest-Good Integration Practices

Install package with pip

For development, we recommend you use
venv for virtual
environments and pip for installing
your application and any dependencies, as well as the pytest package
itself. This ensures your code and dependencies are isolated from your
system Python installation.

Next, place a setup.py file in the root of your package with the
following minimum content:

from setuptools import setup, find_packages

setup(name="PACKAGENAME", packages=find_packages())

Where PACKAGENAME is the name of your package. You can then install
your package in “editable” mode by running from the same directory:

pip install -e .

which lets you change your source code (both tests and application) and
rerun tests at will. This is similar to running
python setup.py develop or conda develop in that it installs your
package using a symlink to your development code.

Conventions for Python test discovery{#test discovery} {#Python test discovery}

pytest implements the following standard test discovery:

  • If no arguments are specified then collection starts from
    testpaths{.interpreted-text role=“confval”} (if configured) or the
    current directory. Alternatively, command line arguments can be used
    in any combination of directories, file names or node ids.
  • Recurse into directories, unless they match
    norecursedirs{.interpreted-text role=“confval”}.
  • In those directories, search for test_*.py or *_test.py files,
    imported by their [test package name](#test package name).
  • From those files, collect test items:
    • test prefixed test functions or methods outside of class
    • test prefixed test functions or methods inside Test prefixed
      test classes (without an __init__ method)

For examples of how to customize your test discovery
example/pythoncollection{.interpreted-text role=“doc”}.

Within Python modules, pytest also discovers tests using the standard
unittest.TestCase <unittest.TestCase>{.interpreted-text role=“ref”}
subclassing technique.

Choosing a test layout / import rules

pytest supports two common test layouts:

Tests outside application code

Putting tests into an extra directory outside your actual application
code might be useful if you have many functional tests or for other
reasons want to keep tests separate from actual application code (often
a good idea):

setup.py
mypkg/
    __init__.py
    app.py
    view.py
tests/
    test_app.py
    test_view.py
    ...

This has the following benefits:

  • Your tests can run against an installed version after executing
    pip install ..
  • Your tests can run against the local copy with an editable install
    after executing pip install --editable ..
  • If you don’t have a setup.py file and are relying on the fact
    that Python by default puts the current directory in sys.path to
    import your package, you can execute python -m pytest to execute
    the tests against the local copy directly, without using pip.

::: {.note}
::: {.title}
Note
:::

See pytest vs python -m pytest{.interpreted-text role=“ref”} for more
information about the difference between calling pytest and
python -m pytest.
:::

Note that this scheme has a drawback if you are using prepend
import mode <import-modes>{.interpreted-text role=“ref”} (which is the
default): your test files must have unique names, because pytest
will import them as top-level modules since there are no packages to
derive a full package name from. In other words, the test files in the
example above will be imported as test_app and test_view top-level
modules by adding tests/ to sys.path.

If you need to have test modules with the same name, you might add
__init__.py files to your tests folder and subfolders, changing them
to packages:

setup.py
mypkg/
    ...
tests/
    __init__.py
    foo/
        __init__.py
        test_view.py
    bar/
        __init__.py
        test_view.py

Now pytest will load the modules as tests.foo.test_view and
tests.bar.test_view, allowing you to have modules with the same name.
But now this introduces a subtle problem: in order to load the test
modules from the tests directory, pytest prepends the root of the
repository to sys.path, which adds the side-effect that now mypkg is
also importable.

This is problematic if you are using a tool like tox to test
your package in a virtual environment, because you want to test the
installed version of your package, not the local code from the
repository.

::: {#src-layout}
In this situation, it is strongly suggested to use a src layout
where application root package resides in a sub-directory of your root:
:::

setup.py
src/
    mypkg/
        __init__.py
        app.py
        view.py
tests/
    __init__.py
    foo/
        __init__.py
        test_view.py
    bar/
        __init__.py
        test_view.py

This layout prevents a lot of common pitfalls and has many benefits,
which are better explained in this excellent blog post by Ionel
Cristian
Mărieș
.

::: {.note}
::: {.title}
Note
:::

The new --import-mode=importlib (see import-modes{.interpreted-text
role=“ref”}) doesn’t have any of the drawbacks above because sys.path
and sys.modules are not changed when importing test modules, so users
that run into this issue are strongly encouraged to try it and report if
the new option works well for them.

The src directory layout is still strongly recommended however.
:::

Tests as part of application code

Inlining test directories into your application package is useful if you
have direct relation between tests and application modules and want to
distribute them along with your application:

setup.py
mypkg/
    __init__.py
    app.py
    view.py
    test/
        __init__.py
        test_app.py
        test_view.py
        ...

In this scheme, it is easy to run your tests using the --pyargs
option:

pytest --pyargs mypkg

pytest will discover where mypkg is installed and collect tests from
there.

Note that this layout also works in conjunction with the src layout
mentioned in the previous section.

::: {.note}
::: {.title}
Note
:::

You can use Python3 namespace packages (PEP420) for your application but
pytest will still perform [test package name](#test package name)
discovery based on the presence of __init__.py files. If you use one
of the two recommended file system layouts above but leave away the
__init__.py files from your directories it should just work on
Python3.3 and above. From “inlined tests”, however, you will need to
use absolute imports for getting at your application code.
:::

::: {#test package name}
::: {.note}
::: {.title}
Note
:::

In prepend and append import-modes, if pytest finds a
"a/b/test_module.py" test file while recursing into the filesystem it
determines the import name as follows:

  • determine basedir: this is the first “upward” (towards the root)
    directory not containing an __init__.py. If e.g. both a and b
    contain an __init__.py file then the parent directory of a will
    become the basedir.
  • perform sys.path.insert(0, basedir) to make the test module
    importable under the fully qualified import name.
  • import a.b.test_module where the path is determined by converting
    path separators / into “.” characters. This means you must
    follow the convention of having directory and file names map
    directly to the import names.

The reason for this somewhat evolved importing technique is that in
larger projects multiple test modules might import from each other and
thus deriving a canonical import name helps to avoid surprises such as a
test module getting imported twice.

With --import-mode=importlib things are less convoluted because pytest
doesn’t need to change sys.path or sys.modules, making things much
less surprising.
:::
:::

tox {#use tox}

Once you are done with your work and want to make sure that your actual
package passes all tests you may want to look into tox, the
virtualenv test automation tool and its pytest
support
. tox
helps you to setup virtualenv environments with pre-defined dependencies
and then executing a pre-configured test command with options. It will
run tests against the installed package and not against your source code
checkout, helping to detect packaging glitches.