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Decorators

  • "Decoration is a way to specify management code for functions and classes." ... "A decorator itself is a callable that returns a callable."
  • A function object is a callable. So, the previous statement can be translated into:
  • A decorator is a function that takes a function object as its argument, and returns a function object, and in the process, makes necessary modifications to the input function, possibly enhancing it.
  • Decorator wraps a function without modifying the function itself. The result of the wrapping?
    • Adds functionality of the function.
    • Modifies the behavior of the function.

Function Decorators

  • Let's start with a function in Python. To understand decorators, we need to know the full scope of capabilities of Python functions.
  • Everything in Python is an object. Function is not an exception.
>>> a=10
>>> def f():
...     pass
...
>>> class MyClass():
...     pass
...
>>> print dir()
['MyClass', '__builtins__', '__doc__', '__name__', '__package__', 'a', 'f']
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  • As we can see, f() is an object, and it's not different from classes (MyClass) or variables (a).
  • We can assign a function to a variable, so the following lines of code is legally perfect.
def func():
   print "func()"

funcObj = func
funcObj()
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  • Functions can be passed around in the same way other types of object such as strings, integers, lists, etc.
  • Another face of a function in Python is it can accept a function as an argument and return a new function object as shown below.
def myFunction(in_function):
   def out_function():
      pass
   return out_function
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  • The myFunction is indeed a decorator because by definition a decorator is a function that takes a function object as its argument, and returns a function object.
  • If we elaborate a little bit more on the function we just defined:
def myFunction(in_function):
   def out_function():
      print "Entry: ", in_function.__name__
      in_function()
      print "Exit: ", in_function.__name__
   return out_function
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How we invoke our decorator ?

  • Let's look at the example below. We put a simple_function into the decorator (myFunction) as an argument, and get a enhanced_function as a return value from the decorator.
def simple_function():
   pass
	
enhanced_function = myFunction(simple_function)
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  • In many cases, we use the same name for the returned function objects as the name of the input function. So, practically, the code should look like this:
def simple_function():
   pass
	
simple_function = myFunction(simple_function)
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  • If we apply the decorator syntax to the code above:
@myFunction
def simple_function():
   pass
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  • Note that the first line @myFunctionas is not a decorator but rather a decorator line or an annotation line, etc.
  • The @ indicates the application of the decorator. A decorator is the function itself which takes a function, and returns a new function. In our case, it is myFunction.
  • When the compiler passes over this code, simple_function() is compiled and the resulting function object is passed to the myFunction code, which does something to produce a function-like object that is then substituted for the original simple_function().
  • Also, note that in the line:
simple_function = myFunction(simple_function)
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  • The decorator(myFunction) is rebinding function name to decorator result.
  • So, when the simple_function is later called, it's actually calling the object returned by the decorator.
  • We've seen the rebinding when we define a static method:
>>> class A:
...    def s(x):
...       print(x)
...    s = staticmethod(s)
... 
>>> A.s(10)
10
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  • The equivalent code using decorator looks like this:
>>> class A:
...    @staticmethod
...    def s(x):
...       print(x)
... 
>>> A.s(10)
10
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  • Another example: suppose we have two functions defined this way:
>>> def wrapper(f):
...    return f
... 
>>> def foo():
...    pass
... 
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  • Then, the wrapper can be used for rebinding foo() like this:
>>> foo = wrapper(foo)
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  • So, it's a decorator:
>>> @wrapper
... def foo():
...    pass
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  • With a decorator defined as below:
def decorator(f):
   #process function
   return f
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  • it automatically maps the following:
@decorator
def f(arg):
   return arg*arg

f(123)  # output 15129
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  • into the equivalent form as shown below:
def f(arg):
   print arg*arg
f = decorator(f)
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  • The decorator is a callable object that returns a callable object with the same number of argument as f.
  • So, decoration maps the following line:
f(123)  
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  • into
decorator(f)(123)    
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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).
 python functions

Learn Python - Python tutorial - python functions - Python examples - Python programs

python - Sample - python code :

# 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("Wikitechy"))

Output:

Welcome to Wikitechy

Function Decorator

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.

decorator examples

We use @func_name to specify a decorator to be applied on another function.

creating decorator

python - Sample - python code :

# 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("Wikitechy") as parameter
print site("Wikitechy")

Output:

Welcome to Wikitechy
simple decorator

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

python - Sample - python code :

# 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.


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