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Decorators

 Decorators are very powerful and useful tool in Python since it allows programmers to modify the behavior of function or class. Decorators allow us to wrap another function in order to extend the behavior of the wrapped function, without permanently modifying it. But before diving deep into decorators let us understand some concepts that will come in handy in learning the decorators.


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("Python"))

Output: "Welcome to python"



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("python")


Output: "Welcome to python"

The above two programs has same results but have 2 different approaches.

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




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