Decorators.pptx

Decorators
• to add functionality to an existing code.
• Metaprogramming as a part of the program
tries to modify another part of the program at
compile time.
• Decorators allow you to make simple
modifications to callable objects like functions,
methods, or classes.

• 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 wrapped
function, without permanently modifying it.
• In Decorators, functions are taken as the
argument into another function and then
called inside the wrapper function

Types
• Two different kinds of decorators in Python:
– Function decorators
– Class decorators

Function Decorators
• 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.

Sample
def decorator_function(org_fun):
def wrapper_function():
print("wrapper executed this before {}".format(org_fun.__name__))
return org_fun()
return wrapper_function
@decorator_function
def display():
print("display functions")
display()

Class Decorators
• If you want to create a callable returning
another callable, the function decorator
approach is easier. If you want the return to
be a function, function decorators should be
preferred, however if you want the decorator
to return a custom object that does something
different to what a function does, in that case
a class decorator should be used

Sample Code
class decorator_class(object):
def __init__(self,org_fun):
self.org_fun=org_fun
def __call__(self):
print("wrapper executed this before {}".format(self.org_fun.__name__))
return self.org_fun)
@decorator_class
def display():
print("display functions")
display()

Chaining Decorators
• A function can be decorated multiple times with different (or
same) decorators.
• The chaining of decorator is similar to how multiple
inheritance can be used to construct classes We can write as
many decorator as we want and include them one by one in
decoration line with decoration syntax before the definition of
function to be decorated.

def makebold(f):
return lambda: "<b>" + f() + "</b>"
def makeitalic(f):
return lambda: "<i>" + f() + "</i>"
@makebold
@makeitalic
def say():
return "Hello"
print say()

Decorator Arguments
• Passing parameters in function
def smart_divide(func):
def inner(a,b):
print("I am going to divide",a,"and",b)
if b == 0:
print("Whoops! cannot divide")
return
return func(a,b)
return inner
@smart_divide
def divide(a,b):
return a/b

def works_for_all(func):
def inner(*args, **kwargs):
print("I can decorate any function")
return func(*args, **kwargs)
return inner
@works_for_all
def divide(a,b):
print(a/b)

• *args (Non Keyword
Arguments)
– Tuple values are
passed inside the
function
def adder(*num):
sum = 0
for n in num:
sum = sum + n
print("Sum:",sum)
adder(3,5)
adder(4,5,6,7)
adder(1,2,3,5,6)
• **kwargs (Keyword
Arguments)
– Dict values are passed
def intro(**data):
print("nData type of
argument:",type(data))
for key, value in
data.items():
print("{} is
{}".format(key,value))
intro(Firstname="Sita",
Lastname="Sharma", Age=22,
Phone=1234567890)
intro(Firstname="John",
Lastname="Wood",
Email="johnwood@nomail.co
m", Country="Wakanda",
Age=25,
Phone=9876543210)**kwarg
s (Keyword Arguments)

def star(func):
print("*" * 30)
func(*args, **kwargs)
print("*" * 30)
return inner
def percent(func):
print("%" * 30)
func(*args, **kwargs)
print("%" * 30)
return inner
@star
@percentdef
printer(msg):
print(msg)
printer("Hello")

class ClassDecorator(object):
def __init__(self, arg1, arg2):
print("Arguements passed to decorator {} and {}".format(arg1, arg2))
self.arg1 = arg1
self.arg2 = arg2
def __call__(self, foo, *args, **kwargs):
def inner_func(*args, **kwargs):
print("Args passed inside decorated function {} and {}".format(self.arg1,
self.arg2))
return foo(*args, **kwargs)
return inner_func
@ClassDecorator("arg1", "arg2")
def print_args(*args):
for arg in args:
print(arg)

Decorators.pptx

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