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)

References

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