closures.md

Python by Example: Closures

A closure is a function that "remembers" variables from its enclosing scope. When you define a function inside another, the inner function can access the outer function's variables even after the outer function returns. Use nonlocal when the inner function needs to modify an outer variable.

What you'll learn:

• Nested functions and scope
• How closures "close over" outer variables
• The nonlocal keyword for reassignment

def make_counter(start=0):
    count = start

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

    return counter


c1 = make_counter()
c2 = make_counter(10)
print(c1())   # 1
print(c1())   # 2
print(c2())   # 11


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


double = make_multiplier(2)
triple = make_multiplier(3)
print(double(5))
print(triple(5))

make_counter returns the inner counter function. Each call to counter increments count and returns it. Without nonlocal, count += 1 would create a local variable instead of modifying the outer one.

To run this program:

$ python source/closures.py
1
2
11
10
15

Tip: Closures are useful for factory functions (like make_counter) and callbacks that need to remember state.

Try it: Write make_adder(n) that returns a function adding n to its argument.

Source: closures.py

Next: Recursion