Test discovery

The Flake8-AAA plugin is triggered for files that look to it like test modules - anything that does not look like a test module is skipped.

The following rules are applied by Flake8-AAA when discovering tests:

  • The module’s filename must start with “test_” and have been collected for linting by Flake8.
  • Every function in the module that has a name that starts with “test” is checked.
  • Test functions can be class methods.
  • Test functions that contain only comments, docstrings or pass are skipped.

These rules are aimed to mirror pytest’s default collection strategy as closely as possible.

If you find that Flake8-AAA is giving false positives (you have checks that you expected to fail, but they did not), then you should check that the plugin did not ignore or skip those tests which you expected to fail.


Flake8-AAA does not check doctests.


For each test found, Flake8-AAA runs the following processes, most of which can be found in Function.check_all().

Check for no-op

Skip test if it is considered “no-op” (pass, docstring, etc).

Mark blank lines

Mark all lines in the test that have no characters and are not part of a string. For example, the following snipped contains only one blank line (line 3 - in the middle of the list), the second at line 9 is part of a string and therefore not counted:

assert result == [

# Check on output
assert str(result) == """[


Find Act node

Search the test for the Act node which will indicate the existence of an Act block. There are four recognised types of Act node:


Action is marked with Marked with # act comment:

do_thing()  # act

Action is wrapped in pytest.raises context manager:

with pytest.raises(ValueError):

Simple result = action:

result = do_thing()

Action is wrapped in unittest’s assertRaises context manager:

with self.assertRaises(ValueError):

Build Act block

The Act node is used to build the Act block for the test. The Act block contains all parent nodes of the Act node up to the test function. For example:

def test():
    with mock.patch('thing.thinger') as mock_thinger:   # < Act block first line
        with mock.patch('other_thing.thinger'):
            with pytest.raises(ValueError):             # < Act node
                do_thing()                              # < Act block last line

    assert mock_thinger.call_count == 0

Build Arrange and Assert blocks

The Arrange block is created with all nodes in the test function that have a line number before the start of the Act block.

The Assert block is created with all nodes in the test function that have a line number after the end of the Act block.

Line-wise analysis

Each block updates a list of line markers for the test and line-wise analysis occurs to ensure that there is one and only one blank line between each block.