Python Closures

A closure is an inner function that remembers the values of the variables of its outer function, even after the outer function has finished running. To understand it, first learn what a nested function is.

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Nested function

A function defined inside another function is a nested (inner) function. It can read the variables of the outer function.

def outer(text):
    def inner():
        print(text)

    inner()

outer("Hello")

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Closure

Now the outer function returns the inner function without calling it (no brackets after inner). We call the returned function later, outside of outer().

def outer(text):
    def inner():
        print(text)

    return inner

my_func = outer("Hello")
my_func()

The outer() function is finished, but my_func still remembers text. That memory is the closure.

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Closure with nonlocal

An inner function can read outer variables, but to change one, you must declare it with the nonlocal keyword. Without it Python thinks you are making a new local variable and raises an UnboundLocalError.

def make_counter():
    count = 0

    def counter():
        nonlocal count
        count += 1
        return count

    return counter

c1 = make_counter()
print(c1())
print(c1())
print(c1())

c2 = make_counter()
print(c2())

Each call of make_counter() makes a new closure with its own count, so c2 starts again from 1.

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Function factory

Closures are good to create many similar functions.

def make_greeting(greeting):
    def say(name):
        print(greeting + ", " + name)
    return say

hello = make_greeting("Hello")
namaste = make_greeting("Namaste")

hello("Robin")
namaste("Maria")

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When to use closures

• To keep some data private without writing a class.
• To create small custom functions, like the greeting example.
• They are the base of decorators.

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Summary

• A closure is an inner function that remembers outer variables.
• Return the inner function without brackets.
• Use nonlocal to change an outer variable.
• Related: Scope of Variables and passing a function as an argument.

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