Background

Following are important facts about functions in Python that are useful to understand decorator functions.
In Python, we can define a function inside another function.
In Python, a function can be passed as parameter to another function (a function can also return another function).

# A Python program to demonstrate that a function
# can be defined inside another function and a
# function can be passed as parameter.

# Adds a welcome message to the string
def messageWithWelcome(str):

# Nested function
def addWelcome():
return "Welcome to "

# Return concatenation of addWelcome()
# and str.
return addWelcome() + str

# To get site name to which welcome is added
def site(site_name):
return site_name

print messageWithWelcome(site("prutor.ai"))
Output:

Welcome to prutor.ai
Function Decorator

A decorator is a function that takes a function as its only parameter and returns a function. This is helpful to “wrap” functionality with the same code over and over again. For example, above code can be re-written as following.
We use @func_name to specify a decorator to be applied on another function.

# Adds a welcome message to the string
# returned by fun(). Takes fun() as
# parameter and returns welcome().
def decorate_message(fun):

# Nested function
def addWelcome(site_name):
return "Welcome to " + fun(site_name)

# Decorator returns a function
return addWelcome

@decorate_message
def site(site_name):
return site_name;

# Driver code

# This call is equivalent to call to
# decorate_message() with function
# site("prutor.ai") as parameter
print site("prutor.ai")
Output:

Welcome to prutor.ai
Decorators can also be useful to attach data (or add attribute) to functions.

# A Python example to demonstrate that
# decorators can be useful attach data

# A decorator function to attach
# data to func
def attach_data(func):
func.data = 3
return func

@attach_data
def add (x, y):
return x + y

# Driver code

# This call is equivalent to attach_data()
# with add() as parameter
print(add(2, 3))

print(add.data)
Output:

5
3
‘add()’ returns sum of x and y passed as arguments but it is wrapped by a decorator function, calling add(2, 3) would simply give sum of two numbers but when we call add.data then ‘add’ function is passed into then decorator function ‘attach_data’ as argument and this function returns ‘add’ function with an attribute ‘data’ that is set to 3 and hence prints it.

Python decorators are a powerful tool to remove redundancy.