How do I detect whether a variable is a function?

python

function-detection

callable

introspection

byAnton Shumikhin·Aug 7, 2024

To figure out if a variable is a function, the most expedient methods involve callable() for a broad scope check or inspect.isfunction() for a more rigorous validation. A quick demonstration:

import inspect

# Defining an "exceptionally" ordinary function! 😉
def example_func(): pass

var = example_func

# Let's see if 'var' has the guts to answer a function call!
is_callable = callable(var) 
# True if 'var' is a suave Python function
is_function = inspect.isfunction(var)  

print(is_callable)  # True
print(is_function)  # True

Understanding nuances of function detection

In the rudimentary case of checking if a variable is a function, implementing callable() and inspect.isfunction() serve the purpose. callable() examines whether an object has callable attributes. It includes user-defined functions, class objects, and objects equipped with the __call__ method. On the other hand, the inspect.isfunction() focuses on user-defined and lambda functions.

Nonetheless, be cautious about relying exclusively on types.FunctionType or raw isinstance validation. They may not be accurate for built-in functions or objects that mimic function behaviors. For Python 3.x users, hasattr(obj, '__call__') can authenticate the presence of the __call__ method without the necessity of the inspect module.

Disassembling callable types

Callable types 101

Establishing a clear understanding of different callable types can evade common pit-holes:

User-defined functions: Received the nod from both callable() and inspect.isfunction().
Lambda functions: They are functionally similar to user-defined functions, but with the privilege of anonymity! They too are identified by inspect.isfunction().
Instances with __call__: Might hoodwink callable() but not inspect.isfunction().
Built-in functions or methods: callable() successfully identifies them, but types.FunctionType may stumble.

Function impersonators

Some objects can pretend to be functions. When a class encapsulates a __call__ method, instances of that class exhibit callable attributes. This confusingly tempting feature enables the creation and use of "funcionettes" - objects that behave like functions but are not!

Diving into the realm of advanced introspection

When you venture into the depth of Python like introspection or bytecode inspection, consider the more intricate tools from the types module. It's similar to your journey into the complex universe inside the simple function structure in Python. However, keep in mind that these scenarios aren't typical in regular function detection.

Sailing with Python trends

With Python's constantly evolving environment, it's vital to follow its idioms and trends. Using callable() aligns with Python's belief in EAFP (easier to ask for forgiveness than permission) by executing an action and then handling exceptions instead of pre-checking types.

To the users fond of type hints and static type checkers like mypy, these tools provide guidance in identifying function usage without the need for runtime checks. Despite static analysis tools not substituting runtime checks, they enrich your code's self-documenting aspect and enhance the developer experience.

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