How to Detect Whether a Variable Represents a Function in Python?

Detecting Whether a Variable is a Function

In Python, determining if a variable represents a function can be perplexing. The intuitive approach, isinstance(x, function), fails due to the undefined 'function' reference.

Solution 1: callable()

For Python 2.x or 3.2 , callable() provides a direct and straightforward check:

<code class="python">callable(x)</code>

Solution 2: call Attribute (Python 3.x Before 3.2)

Prior to Python 3.2, one must inspect the object for the __call__ attribute:

<code class="python">hasattr(x, '__call__')</code>

Caveats with Types and Inspection

While types.FunctionTypes and inspect.isfunction methods exist, they may provide unexpected results. Non-Python functions, such as many builtins, return False using these methods:

<code class="python">import types, inspect
isinstance(open, types.FunctionType) # False
callable(open) # True</code>

Best Practice: Duck Typing

Rather than relying on isinstance or inspect, the preferred approach is to check if the object can be called, mimicking the "duck-typed" principle:

<code class="python">def quacks_like_a_function(x):
    return callable(x)</code>

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