B902: don't false-positive on metaclasses with a dotted base

[StressTestor]

what

B902 raises a false positive on a metaclass whose base is written with a dotted name, e.g.

import abc

class CustomMeta(abc.ABCMeta):
    def __new__(mcs, name, bases, ns): ...  # flagged, but mcs is correct here

This is issue #411.

why

The B902 metaclass check builds the set of base names from bare ast.Name nodes only:

bases = {b.id for b in cls.bases if isinstance(b, ast.Name)}

A dotted base like abc.ABCMeta is an ast.Attribute, so it gets dropped. The class is then treated as a plain class and its cls/metacls/mcs first arguments are flagged. #415 added detection for the bare type/ABCMeta/EnumMeta names back in 2023; the dotted form was the remaining gap, and #411 is still open.

fix

Also take the attribute name from dotted ast.Attribute bases:

bases = {
    b.id if isinstance(b, ast.Name) else b.attr
    for b in cls.bases
    if isinstance(b, (ast.Name, ast.Attribute))
}

Scoped to direct bases (transitive base chains stay out of scope). This is the same name-based heuristic the check already used, so dotted bases now behave like the bare ones.

tests

tests/eval_files/b902.py gains dotted abc.ABCMeta and enum.EnumMeta metaclasses (clean), plus a precision case where a wrong first arg on a dotted metaclass is still reported as a metaclass instance method. Before the fix those clean fixtures emit spurious B902s and the precision error is lost; after, they are correct.

Two classes in tests/eval_files/b024.py (multi_super_1, non_keyword_abcmeta_2) directly subclass abc.ABCMeta and used self, which was only valid while the bug hid their metaclass-ness. They now use cls, consistent with the bare non_keyword_abcmeta_1 sibling in the same file.

Also updated the B902 README entry, which only mentioned type, to cover ABCMeta/EnumMeta and dotted bases. Full suite green.

Fixes #411.

[Copilot]

Pull request overview

Fixes a B902 false-positive where custom metaclasses inheriting from dotted standard-library metaclass names (e.g. abc.ABCMeta, enum.EnumMeta) were not recognized as metaclasses, causing valid first-argument names like mcs/metacls/cls to be incorrectly flagged.

Changes:

• Extend B902 metaclass-base detection to include ast.Attribute bases (dotted bases), not just bare ast.Name.
• Add/adjust eval fixtures to ensure dotted metaclass bases don’t trigger B902, while still reporting genuinely-wrong first arguments.
• Update README B902 documentation and changelog entry to reflect the dotted-base behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
bugbear.py	Updates B902 base-name extraction to include dotted bases (ast.Attribute) so metaclasses are correctly detected.
tests/eval_files/b902.py	Adds fixtures covering dotted abc.ABCMeta / enum.EnumMeta metaclasses (no warnings) and a precision case (still warns).
tests/eval_files/b024.py	Adjusts two fixtures to use cls so they remain clean under the corrected B902 behavior.
README.rst	Updates B902 description and UNRELEASED changelog to document the dotted-base fix.

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[cooperlees]

Many Thanks! Nice.