Function Decorators in Python | Set 1 (Introduction)

Background

Following are important facts about functions in Python that are useful to understand decorator functions.

  1. In Python, we can define a function inside another function.
  2. 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("GeeksforGeeks"))

Output:

Welcome to GeeksforGeeks

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("GeeksforGeeks") as parameter
print site("GeeksforGeeks")

Output:

Welcome to GeeksforGeeks

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.

This article is contributed by Shwetanshu Rohatgi. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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