pytest-param-scope

The pytest parametrize scope fixture workaround.

pytest-param-scope provides a param_scope marker to pass setup and teardown functions to a parametrized function.

There's also a param_scope fixture to allow the return value of the setup function to be passed to the test.

Installation

From PyPI:

$ pip install pytest-param-scope

Example

import pytest


def param_setup():
    print('\nparam setup')
    return 'some data from setup'


def param_teardown():
    print('\nparam teardown')


@pytest.mark.param_scope(param_setup, param_teardown)
@pytest.mark.parametrize('x', ['a', 'b', 'c'])
def test_param(x, param_scope):
    """
    mark.param_scope is used to pass
    setup and teardown functions to fixture

    param_scope fixture is necessary if you
    want to use the value from setup
    """
    assert param_scope == 'some data from setup'

Let's see it run:

(venv) $ pytest -s -v test_param_scope.py::test_param
================== test session starts ===================
collected 3 items                                        

test_param_scope.py::test_param[a] 
param setup
PASSED
test_param_scope.py::test_param[b] PASSED
test_param_scope.py::test_param[c] PASSED
param teardown

=================== 3 passed in 0.01s ====================

What are we seeing?

1. Setup is run before the first parameter in a set.
2. Teardown is run after the last parameter.
3. The param_scope fixture holds the return value of the setup function.

Similarities to Fixtures

• Teardown is not run if the setup fails.
• Setup is run once, even though the value can be retrieved by all parametrized test cases.
• If an exception occurs in setup, the test will report Error and not run. The teardown will also not run.
• If an exception occurs in teardown, the LAST parametrized test case to run results in BOTH PASS and Error. This is weird, but consistent with pytest fixtures.

You can combine setup and teardown in one function

You can provide a function separated by a yield to put both setup and teardown in one function.

However, there's a trick to doing this:

• Either, pass None as the teardown.
• Or use with_args, as in @pytest.mark.param_scope.with_args(my_func)

Here's a combo setup/teardown function:

def setup_and_teardown():
    print('\nsetup')
    yield 42
    print('\nteardown')

Calling it with None for teardown:

import pytest

@pytest.mark.param_scope(setup_and_teardown, None)
@pytest.mark.parametrize('x', ['a', 'b', 'c'])
def test_yield(x, param_scope):
    assert param_scope == 42

Or using with_args:

@pytest.mark.param_scope.with_args(setup_and_teardown)
@pytest.mark.parametrize('x', ['a', 'b', 'c'])
def test_just_one_func(x, param_scope):
    assert param_scope == 42

Both of these examples are in examples/test_yield.py.

More examples

Please see examples directory in the repo.

Limitations

The implementation is a total hack and relies on global variables and looking up the next test to see when to run the teardown. There is undoubtedly room for improvement.

• With pytest-xdist: I haven't come up with a failing case yet, but it seems like this will be sketchy with tests running in parallel.

• With pytest-repeat: This actually works great with pytest-repeat, as repeat works by generating parametrized tests.

FAQ

Why not just use existing fixture scopes?

There isn't a scope that quite fits.

• Function: runs setup before and after each parametrized test case.
• Class: Kinda works if you put only one function in a test class.
• Module: Too wide.
• Session: Way too wide.

I want a setup that runs before all parametrized test cases, and clean up after the last one.

Why implement this as a plugin and not add this functionality to pytest core?

A couple reasons.

1. I'm not sure we want this funcitonality in core pytest. Playing with it as a plugin will tell us if it's important to people.
2. Implementing it as a plugin is faster to get it out there and usable.

Contributing

Contributions are very welcome. Tests can be run with tox.

License

Distributed under the terms of the MIT license, "pytest-param-scope" is free and open source software

Issues

If you encounter any problems, please file an issue along with a detailed description.

Changelog

See changelog.md