Python - Decorators - Nested Functions - Extend Functions

Decorators

Decorators provide a way to modify functions using other functions. 
This is ideal when you need to extend the functionality of functions that you don't want to modify.
Example:

def decor(func):
def wrap():
print("============")
func()
print("============")
return wrap

def print_text():
print("Hello world!")

decorated = decor(print_text)
decorated()

We defined a function named decor that has a single parameter func. Inside decor, we defined a nested function named wrap. The wrap function will print a string, then call func(), and print another string. The decor function returns the wrap function as its result.
We could say that the variable decorated is a decorated version of print_text - it's print_text plus something. 
In fact, if we wrote a useful decorator we might want to replace print_text with the decorated version altogether so we always got our "plus something" version of print_text. 
This is done by re-assigning the variable that contains our function:

print_text = decor(print_text)
print_text()

Now print_text corresponds to our decorated version.

Python provides support to wrap a function in a decorator by pre-pending the function definition with a decorator name and the @ symbol. 
If we are defining a function we can "decorate" it with the @ symbol like:

@decor
def print_text():
print("Hello world!")