unreliable-callable-check (B004)

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 if there's comments in the hasattr call expression, as comments may be removed.

For example, the fix would be marked as unsafe in the following case:

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

References

• Python documentation: callable
• Python documentation: hasattr
• Python documentation: __getattr__
• Python documentation: __call__