title | description |
---|---|
Python Decorators - Python Cheatsheet |
A Python Decorator is a syntax that provide a concise and reusable way for extending a function or a class. |
A Python Decorator provides a concise and reusable way for extending a function or a class.
A decorator in its simplest form is a function that takes another function as an argument and returns a wrapper. The following example shows the creation of a decorator and its usage.
def your_decorator(func):
def wrapper():
# Do stuff before func...
print("Before func!")
func()
# Do stuff after func...
print("After func!")
return wrapper
@your_decorator
def foo():
print("Hello World!")
foo()
# Before func!
# Hello World!
# After func!
def your_decorator(func):
def wrapper(*args,**kwargs):
# Do stuff before func...
print("Before func!")
func(*args,**kwargs)
# Do stuff after func...
print("After func!")
return wrapper
@your_decorator
def foo(bar):
print("My name is " + bar)
foo("Jack")
# Before func!
# My name is Jack
# After func!
This template is useful for most decorator use-cases. It is valid for functions with or without parameters, and with or without a return value.
import functools
def your_decorator(func):
@functools.wraps(func) # For preserving the metadata of func.
def wrapper(*args,**kwargs):
# Do stuff before func...
result = func(*args,**kwargs)
# Do stuff after func..
return result
return wrapper
You can also define parameters for the decorator to use.
import functools
def your_decorator(arg):
def decorator(func):
@functools.wraps(func) # For preserving the metadata of func.
def wrapper(*args,**kwargs):
# Do stuff before func possibly using arg...
result = func(*args,**kwargs)
# Do stuff after func possibly using arg...
return result
return wrapper
return decorator
To use this decorator:
@your_decorator(arg = 'x')
def foo(bar):
return bar
A decorator can also be defined as a class instead of a method. This is useful for maintaining and updating a state, such as in the following example, where we count the number of calls made to a method:
class CountCallNumber:
def __init__(self, func):
self.func = func
self.call_number = 0
def __call__(self, *args, **kwargs):
self.call_number += 1
print("This is execution number " + str(self.call_number))
return self.func(*args, **kwargs)
@CountCallNumber
def say_hi(name):
print("Hi! My name is " + name)
say_hi("Jack")
# This is execution number 1
# Hi! My name is Jack
say_hi("James")
# This is execution number 2
# Hi! My name is James