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subclass-builtin (FURB189)

Preview (since 0.7.3) · Related issues · View source

Derived from the refurb linter.

Fix is always available.

This rule is unstable and in preview. The --preview flag is required for use.

What it does

Checks for subclasses of dict, list or str.

Why is this bad?

Built-in types don't consistently use their own dunder methods. For example, dict.__init__ and dict.update() bypass __setitem__, making inheritance unreliable.

Use the UserDict, UserList, and UserString objects from the collections module instead.

Example

class UppercaseDict(dict):
    def __setitem__(self, key, value):
        super().__setitem__(key.upper(), value)


d = UppercaseDict({"a": 1, "b": 2})  # Bypasses __setitem__
print(d)  # {'a': 1, 'b': 2}

Use instead:

from collections import UserDict


class UppercaseDict(UserDict):
    def __setitem__(self, key, value):
        super().__setitem__(key.upper(), value)


d = UppercaseDict({"a": 1, "b": 2})  # Uses __setitem__
print(d)  # {'A': 1, 'B': 2}

Fix safety

This fix is marked as unsafe because isinstance() checks for dict, list, and str types will fail when using the corresponding User class. If you need to pass custom dict or list objects to code you don't control, ignore this check. If you do control the code, consider using the following type checks instead:

  • dict -> collections.abc.MutableMapping
  • list -> collections.abc.MutableSequence
  • str -> No such conversion exists

References