Closures

A nested scope/function is where a function is held within another function. The inner function could be making use of free variables in the outer function as part of its routine.

def main_function():
    msg = 'Hello' # Free Variable
    def inner_function():
        print (msg) # Request for Free Variable
    inner_function()
    print (msg)
    
main_function()

The order that the commands are inside a function matters.

In the following, the msg variable can be seen in two locales.

The main_function and the inner_function.

msg of the inner_function is not seen by the main_function. The inner_function msg is a local variable. When it goes to print, the inner variable is assigned the value of the free variable and the inner_function does not look outside the main_function.

The first print in this function will be actioned by the inner_function() and the main_function print statement will action after

def main_function():
    msg = 'Vincent'
    def inner_function():
        msg = 'Hello'
        print (msg)
    inner_function()
    print(msg)

main_function()

The following code shows another possible way of changing one of the variables.

msg is initial given the value 'Vincent' as the main_function starts. As it progresses into the inner_function, the non-local msg commands then states that msg is a variable outside of the function. This means when the variable msg is changed to 'Hello' by the inner_function it changes the value outside the inner function as well.

Both the main_function and the inner_function will print the same value of 'Hello'

def main_function():
    msg = 'Vincent'
    def inner_function():
        nonlocal  msg
        msg = 'Hello'
        print (msg)
    inner_function()
    print(msg)

main_function()

Instead of the function printing out values to the console the inner_function is returning a callable value to the main_function. The main_function that is callable is then assigned to a variable object. This allows the variable to be treated as the main_function.

def main_function():
    msg = 'Mars Attacks'
    def inner_function():
        print (msg)
    return inner_function

object = main_function()

Adding the parenthesis to the object will action the main_function and print the msg.

object()
Mars Attacks