First-class functions & closures

Functions that remember

Lesson 3 of 20 · 10 min · Module 1: Functions, deeper

In Python, functions are values. You can store them, pass them to other functions, and create new ones inside a function. An inner function can remember the variables around it.

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By the end, you’ll be able to…

Treat functions as values
Create a closure
Change an enclosing variable with nonlocal

The idea

A closure is an inner function that keeps access to variables of the function that created it, even after that function has returned. To change such a variable from the inner function, declare it with nonlocal.

def make_multiplier(factor):
    def multiply(x):
        return x * factor
    return multiply

triple = make_multiplier(3)
print(triple(5))

01Functions are objects

sorted(names, key=len) passes the function len as a value. You can also return a function from another function, as make_multiplier does.

02Closures remember

Each call of make_counter creates a new current variable and a new inner function bound to it. That is why two counters do not interfere.

def make_counter():
    count = 0
    def bump():
        nonlocal count
        count += 1
        return count
    return bump

a = make_counter()
print(a(), a())

Output

1 2

03Why nonlocal is needed

Assigning to a name inside a function normally creates a new local variable. nonlocal tells Python to update the variable from the enclosing function instead.

Your turn

Write make_counter(start=0, step=1) that returns a function. Each call of the returned function returns the next number: first start, then start + step, and so on. Two counters must count independently.

What the checks look for

Counts from the start value
Uses a custom start and step
Two counters are independent

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Take this with you

A closure is a small object made of a function plus the variables it remembers.

Go a little further

Add a reset() function by returning a tuple of two closures that share the same variable.