Pass function as an argument in Python

In Python a function is an object, just like a number or a string. So you can store a function in a variable, pass it to another function as an argument, and even return it from a function. A function that takes another function as an argument is called a higher order function.

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Functions are objects

You can give a function another name by assigning it to a variable. Notice that we write the function name without brackets, because brackets would call it.

def display(text):
    return text.upper()

print(display("Hello"))

show = display
print(show("Hello"))

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Pass a function to another function

Here the function meet() takes a function as its argument and calls it.

def show(text):
    return text.upper()

def display(text):
    return text.lower()

def meet(func):
    greeting = func("This is created by a function passed as an argument.")
    print(greeting)

meet(show)
meet(display)

Same meet() gives different results, depending on which function we give it.

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More examples

def apply(func, value):
    return func(value)

print(apply(len, "python"))
print(apply(str.upper, "python"))

def apply_twice(func, x):
    return func(func(x))

print(apply_twice(lambda n: n * 2, 3))

The built-in functions sorted(), map() and filter() are higher order functions too. For example, sorted(words, key=len) sorts the words by their length.

words = ["banana", "fig", "apple"]
print(sorted(words, key=len))

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Return a function from a function

A function can also create and return another function. This idea is used in closures and decorators.

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Summary

• Functions are objects, so they can be stored in variables and passed around.
• Pass the function name without brackets.
• A function that takes or returns another function is a higher order function.
• Related: lambda and closures.

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