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pytest-patch-with-lambda (PT008)#

Derived from the flake8-pytest-style linter.

What it does#

Checks for mocked calls that use a dummy lambda function instead of return_value.

Why is this bad?#

When patching calls, an explicit return_value better conveys the intent than a lambda function, assuming the lambda does not use the arguments passed to it.

return_value is also robust to changes in the patched function's signature, and enables additional assertions to verify behavior. For example, return_value allows for verification of the number of calls or the arguments passed to the patched function via assert_called_once_with and related methods.


def test_foo(mocker):
    mocker.patch("", lambda x, y: 7)

Use instead:

def test_foo(mocker):
    mocker.patch("", return_value=7)

    # If the lambda makes use of the arguments, no diagnostic is emitted.
    mocker.patch("module.other_target", lambda x, y: x)