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unreliable-callable-check (B004)

Added in v0.0.106 · Related issues · View source

Derived from the flake8-bugbear linter.

Fix is sometimes available.

What it does

Checks for uses of hasattr to test if an object is callable (e.g., hasattr(obj, "__call__")).

Why is this bad?

Using hasattr is an unreliable mechanism for testing if an object is callable. If obj implements a custom __getattr__, or if its __call__ is itself not callable, you may get misleading results.

Instead, use callable(obj) to test if obj is callable.

Example

hasattr(obj, "__call__")

Use instead:

callable(obj)

Fix safety

This rule's fix is marked as unsafe because the replacement may not be semantically equivalent to the original expression, potentially changing the behavior of the code.

For example, an imported module may have a __call__ attribute but is not considered a callable object:

import operator

assert hasattr(operator, "__call__")
assert callable(operator) is False

Additionally, __call__ may be defined only as an instance method:

class A:
    def __init__(self):
        self.__call__ = None


assert hasattr(A(), "__call__")
assert callable(A()) is False

Additionally, if there are comments in the hasattr call expression, they may be removed:

hasattr(
    # comment 1
    obj,  # comment 2
    # comment 3
    "__call__",  # comment 4
    # comment 5
)

References