6.1. Decorator About

  • Decorator is an object, which takes another object as it's argument

  • Since Python 2.4: PEP 318 -- Decorators for Functions and Methods

  • Since Python 3.9: PEP 614 -- Relaxing Grammar Restrictions On Decorators

  • Decorator can do things before call

  • Decorator can do things after call

  • Decorator can modify arguments

  • Decorator can modify returned value

  • Decorator can avoid calling

  • Decorator can modify globals

  • Decorator can add or change metadata

../../_images/decorator-about-call.png

Decorator is an object (such as function, method or class) that modifies the behavior of another object. It takes a decorated object as input and returns a modified version of that object. Decorators are a powerful feature that allows you to add functionality to an object without modifying its source code.

6.1.1. Syntax

  • func is a reference to function which is being decorated

  • args arbitrary number of positional arguments

  • kwargs arbitrary number of keyword arguments

  • By calling func(*args, **kwargs) you actually run original (wrapped) function with it's original arguments

Decorators are defined using the "@" symbol followed by the name of the decorator. When a function is decorated, the decorator is called with the original object as its argument. The decorator can then modify the behavior of the original object by adding new functionality or modifying its existing behavior.

Here is an example of a simple decorator that adds an additional behavior to a function:

>>> def mydecorator(func):
...     def wrapper():
...         print('before')
...         result = func()  # call the original function
...         print('after')
...         return result
...     return wrapper
>>>
>>> @mydecorator
... def myfunction():
...     print('This is my function.')
>>>
>>> myfunction()
before
This is my function.
after

In this example, the mydecorator function takes a function as input and returns a new function wrapper that prints a trext before and after calling the original function. The @mydecorator syntax is used to apply the decorator to the myfunction function. When myfunction is called, it will now print "before", execute its original code, and then print "after".

6.1.2. Decoration

Syntax:

>>> @mydecorator
... def myfunction(*args, **kwargs):
...     ...

Is equivalent to:

>>> myfunction = mydecorator(myfunction)