A closure is a nested function that preserves the context of the outer function even after it completes execution.

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


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

print(double(10))  # 20
print(triple(10))  # 30

The multiply function is a closure because:

• it is defined within make_multiplier;
• it uses the factor variable from the outer function;
• it continues to have access to factor after make_multiplier completes.

Each call creates a separate environment. Therefore, a function created by make_multiplier(2) retains access to factor = 2, while one created by make_multiplier(3) retains access to factor = 3.

This feature in Python allows you to implement decorators, function factories, and stateful functions.

The interviewer is checking whether the candidate understands:

• variable scopes;
• nested functions;
• lexical scoping;
• variable lifetime;
• functions as first-class objects;
• how decorators and function factories work;
• Python’s late-binding behavior.

The question also reveals whether the candidate can explain how a function can preserve state without using a global variable or a class.

The candidate should explain that a closure:

• is a function;
• refers to a variable from an enclosing scope;
• retains access to that variable after the outer function has returned.