How to Check If a Function Exists in Python

In dynamic programming environments or when loading modules at runtime, you may need to verify if a function exists before calling it. Calling an undefined function raises a NameError or AttributeError, crashing your script.

This guide explains how to check for function existence using the in operator, hasattr(), and callable().

Method 1: Checking Local/Global Scope

If the function is defined in your current script, its name resides in either the locals() or globals() dictionary.

def greet():
    print("Hello!")

if "greet" in globals():
    print("Function 'greet' exists.")
    greet()
else:
    print("Function not found.")

if "goodbye" not in globals():
    print("Function 'goodbye' does not exist.")

Output:

Function 'greet' exists.
Hello!
Function 'goodbye' does not exist.

Method 2: Using hasattr() for Modules and Classes

When working with imported modules or objects, hasattr() is the standard way to check for the existence of functions (which are attributes of the module/object).

Checking a Module

import math

if hasattr(math, "sqrt"):
    print(f"sqrt(16) = {math.sqrt(16)}")
else:
    print("math.sqrt not found.")

Output:

sqrt(16) = 4.0

Checking an Object

class Robot:
    def speak(self):
        print("Beep boop.")

bot = Robot()

if hasattr(bot, "speak"):
    bot.speak()

Output:

sqrt(16) = 4.0
Beep boop.

Method 3: Verifying Callability with callable()

Just because an attribute exists doesn't mean it's a function. It could be a variable (integer, string, etc.). To ensure you can actually execute it, use callable().

def my_func():
    return "I run!"

my_var = 42


if callable(my_func):
    print("my_func is callable.")

if not callable(my_var):
    print("my_var exists but is NOT callable.")

Output:

my_func is callable.
my_var exists but is NOT callable.

Conclusion

To check if a function exists in Python:

1. Use name in globals() for functions defined in the current script.
2. Use hasattr(obj, "name") for functions inside modules or classes.
3. Use callable(obj) to verify that the object is actually a function and not a variable.