Can anyone explain the code?

[MPython]

How it works?

def externa(x):
    def interna(y):
        return y**x
    return interna
f1=externa(2)
print(f1(3)) #----> 9
print(f1(4))#----> 16
print('............')
f2=externa(4)
print(f2(3))#----> 81
print(f2(4))#----> 256

[DeaD_EyE]

This is a closure. In Python functions are first class function. A function inside a function have access to variables which are defined in the outer function. A function can return a function.

def foo():
    # local variable in function
    # foo
    greeting = 'Hello World'
    def bar():
        # access to local variable
        # greeting of
        # function foo
        print(greeting)
    # return the function
    # bar
    # without calling it
    return bar

When you call foo(), it returns the function bar. Then you can call the returned function, which has still access to the variable greeting.

inner_function = foo()
inner_function() => prints Hello World

or

foo()()

The statement def is an assignment for a function to a name.
You can write it with a lambda:

function_name = lambda x: x ** 2
function_name.__name__ = 'function_name'

Written with lambda with your example:

def externa(x):
    interna = lambda y: y**x
    # the variable x is not in the
    # function definition of interna
    # it's in the function definition of externa
    return interna
    # this returns interna
    # but does it not call it

I think you should have enough knowledge to understand what happens.
Outer function returns inner function. Inner function has access local variables of outer function.
A function can return a function. You can return a function without calling the function.

[ichabod801]

Never mind

[wavic]

You should see about the decorators. A decorator is a function which wraps another function and takes as an argument the function you want to decorate.

>>> def decorator(func):
...    
...     def wrapper(text):
...         print('Before')
...         func(text)
...         print('After')
...     return wrapper

>>> @decorator
... def say(text):
...     print(text)

>>> say('Hello')
Before
Hello
After

This way you can change the behaviour of any function and what it's doing.