What is a closure?

A closure is a nested function that remembers variables from its enclosing scope even after that outer function has returned.

Those remembered names live in the inner function's __closure__ cells. Closures power decorators, callbacks, and lightweight function factories.

def make_multiplier(factor):
    def multiply(x):
        return x * factor   # "factor" is a free variable
    return multiply

double = make_multiplier(2)
triple = make_multiplier(3)
print(double(10))  # 20
print(triple(10))  # 30

# Late-binding pitfall with loops
funcs = []
for i in range(3):
    funcs.append(lambda: i)          # all see final i
print([f() for f in funcs])          # [2, 2, 2]

funcs = []
for i in range(3):
    funcs.append(lambda i=i: i)      # bind current i as default
print([f() for f in funcs])          # [0, 1, 2]

# Mutating enclosed state with nonlocal
def counter():
    n = 0
    def inc():
        nonlocal n
        n += 1
        return n
    return inc

c = counter()
print(c(), c())  # 1 2

Interview tip: Define closure as "function + captured enclosing state," then mention nonlocal and the late-binding loop gotcha.