What is a closure in Python?

A closure is a function that remembers the values from its surrounding scope even after that scope no longer exists.

Key features:

• Nested function: A closure occurs when a function is defined inside another function.
• Access to outer variables: The nested function accesses one or more variables from its containing outer function.
• Preservation of state: The values of these outer variables are preserved in the closure after the outer function has finished executing.

Example:

def outer_function(x):
    def inner_function(y):
        return x + y  # inner_function uses the variable x from outer_function
    return inner_function

# Creating a closure by passing a value for x
closure_add_five = outer_function(5)

# Calling the closure with a value for y
result = closure_add_five(3)
# The variable x (with value 5) is stored in closure_add_five

In this example, inner_function is a closure. It "remembers" the value of the variable x (which was 5 at the time outer_function was called), even after outer_function has finished. When calling closure_add_five(3), the closure uses the stored value x=5 and computes 5 + 3.

Uses of closures:

• Implementation of decorators.
• Partial application of functions.
• Creating functional objects with preserved state.