What causes the creation of a closure in a Python function and how does it work?

A closure in Python occurs when an inner function references variables from the outer function's scope, and the outer function returns this inner function. The closure preserves the state of these variables even after the outer function has finished executing.

Example:

def outer(x):
    def inner(y):
        return x + y
    return inner

f = outer(10)
print(f(5))  # Outputs 15

Here, inner is a closure that remembers the value of x from outer. This allows creating functions with a preserved context.