PyCQA / PyCQA/pycodestyle

E721: pycodestyle v2.11.0 does not raise the error to `int == type(obj)`

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Description

Environment
% python -V
Python 3.11.4
% pycodestyle --version
2.11.0

% pip list
Package     Version
----------- -------
pip         23.2.1
pycodestyle 2.11.0
setuptools  68.0.0
Normal behavior
% cat example.py
if type(obj) == int:
    pass
% pycodestyle example.py
example.py:1:4: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`

Great!

Strange behavior

Change type(obj) == int to int == type(obj) (swap the left and right sides).

Expected

Raise E721.
This code is still comparing the return value of type.

Actual

Do NOT raise E721.

% cat example.py
if int == type(obj):
    pass
% pycodestyle example.py
% echo $?
0
Investigation
>>> from pycodestyle import comparison_type
>>> list(comparison_type("if type(obj) == int:", noqa=False))
[(3, 'E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`')]
>>> list(comparison_type("if int == type(obj):", noqa=False))
[]

In v2.11.0 (21abd9b6dcbfa38635bc85a2c2327ec11ad91ffc), COMPARE_TYPE_REGEX has 2 captures.
https://github.com/PyCQA/pycodestyle/blob/21abd9b6dcbfa38635bc85a2c2327ec11ad91ffc/pycodestyle.py#L128-L131

In comparison_type, the first captured substring is only referred (inst = match.group(1)).
https://github.com/PyCQA/pycodestyle/blob/21abd9b6dcbfa38635bc85a2c2327ec11ad91ffc/pycodestyle.py#L1451-L1452
https://github.com/PyCQA/pycodestyle/blob/21abd9b6dcbfa38635bc85a2c2327ec11ad91ffc/pycodestyle.py#L1461-L1470

>>> from pycodestyle import COMPARE_TYPE_REGEX
>>> from pycodestyle import SINGLETONS

>>> match = COMPARE_TYPE_REGEX.search("if type(obj) == int:")
>>> match
<re.Match object; span=(3, 15), match='type(obj) =='>
>>> match.group(1)  # None; Go yield (L1466)
>>> match.group(2)
'obj'

>>> match = COMPARE_TYPE_REGEX.search("if int == type(obj):")
>>> match
<re.Match object; span=(7, 19), match='== type(obj)'>
>>> match.group(1)
'obj'
>>> match.group(2)
>>> inst = match.group(1)
>>> inst and inst.isidentifier() and inst not in SINGLETONS  # Go return (L1465) not yield
True

If you need to use captured substrings, you might need to do something like inst = match.group(1) or match.group(2).
However, in that case, inst and inst.isidentifier() and inst not in SINGLETONS would return True, and E721 would no longer be raised.
Therefore, I believe that removing the return statement here would resolve the reported issue.

One concern I have is that it seems inst = match.group(1) has not been changed along with the last pull request.
https://github.com/PyCQA/pycodestyle/pull/1086/files
Depending on the reason for not changing this, we might need to consider a different solution.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start in pycodestyle.py with COMPARE_TYPE_REGEX and comparison_type, then reproduce both comparison forms using the commands shown in the issue. Adjust the handling so int == type(obj) raises E721 while the existing type(obj) == int behavior remains correct, and verify both cases with pycodestyle.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
tooling
Issue type
Bug
Difficulty
2/5
Estimated time
1-3 hours
Activity status
Stale
Clarity
Clearly specified
Newbie friendliness
48/100

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