Adjust behaviour of context manager analysis

[jamescooke]

Current behaviour

As of v0.9.0, Flake8-AAA considers that all with statements wrapping the Act block are part of that Act block. So for a simple test using open() this is the current required layout:

def test():
    with open('f.txt') as f:
        result = f.read()

    assert result == 'Hello World!\n'

Analysis shows that the with open() statement has been absorbed into the Act block:

$ python -m flake8_aaa test.py 
------+------------------------------------------------------------------------
 1 DEF|def test():
 2 ACT|    with open('f.txt') as f:
 3 ACT|        result = f.read()
 4 BL |
 5 ASS|    assert result == 'Hello World!\n'
------+------------------------------------------------------------------------
    0 | ERRORS
======+========================================================================
        PASSED!

Doing an open() on a file is an Arrange operation - it's preparing for the test. Therefore it should be part of the Arrange block. That means that the Act block just consists of the result assignment line result = f.read().

Now for the layout - the README states:

each test is broken down into three distinct parts separated by blank lines.

Therefore there must be a blank line before the result assignment. It is inside the with statement block and the test is laid out as:

def test():
    with open('f.txt') as f:

        result = f.read()

    assert result == 'Hello World!\n'

However, using this layout with the current version of Flake8-AAA gives an error - analysis shows:

$ python -m flake8_aaa test.py 
------+------------------------------------------------------------------------
 1 DEF|def test():
 2 ACT|    with open('f.txt') as f:
 3 BL |
       ^ AAA05 blank line in block
 4 ACT|        result = f.read()
 5 BL |
 6 ASS|    assert result == 'Hello World!\n'
------+------------------------------------------------------------------------
    1 | ERROR
======+========================================================================
        FAILED with 1 ERROR

Expected behaviour

No error is raised from the example above, analysis gives line 2 as an Arrange block:

$ python -m flake8_aaa test.py 
------+------------------------------------------------------------------------
 1 DEF|def test():
 2 ARR|    with open('f.txt') as f:
 3 BL |
 4 ACT|        result = f.read()
 5 BL |
 6 ASS|    assert result == 'Hello World!\n'
------+------------------------------------------------------------------------
    0 | ERRORS
======+========================================================================
        PASSED!

Checklist

• Remove Act block building code that vacuums up parent with statements.

• Add new examples to good files: with in Arrange, with in Assert, with spanning whole test

• Add bad examples for: non-assertive with wrapping Act (assignment and raises types).

• Run pytest against new / edited good and bad examples. All examples should be executable tests and passing.

• Check how black likes to format with statements - there may be incompatibility.

• Update documentation on "Build Act Block" https://flake8-aaa.readthedocs.io/en/stable/discovery.html#build-act-block

• Update CHANGELOG