How to Determine if a Variable Represents a Function in Python

In Python, it can be useful to check if a variable points to a function. Initially, one might consider using isinstance(x, function), but Python raises a NameError for the undefined function class.

Callable Objects and hasattr

To determine if a variable is a function, Python offers callable(). For Python 2.x and Python 3.2 , this method provides the functionality to check for callable objects. Alternatively, for Python 3.x prior to 3.2, one can use hasattr(obj, '__call__') to examine if the object possesses a __call__ attribute characteristic of callable objects.

Caveats with types.FunctionType and inspect.isfunction

While approaches utilizing types.FunctionTypes or inspect.isfunction have been suggested, these methods have limitations. They return False for non-Python functions. Built-in functions, implemented in C, fail this check, even though they are callable:

>>> isinstance(open, types.FunctionType)
False
>>> callable(open)
True

Therefore, verifying the callable nature of an object should involve explicitly checking its ability to be called, not its conformance to a specific class definition.