Decorator

Decorator Design Pattern in Python

Decorator Design Pattern in Python

Design Patterns Home

What is it?

The Decorator Design Pattern gives us a method to add new functionality to an object, dynamically. Python makes implementing this pattern relatively easier as compared to other languages, due to its builtin feature called decorators. It is a Structural Design Pattern as it eases the application design by identifying a simple way to add features to an existing object dynamically.

Why the need for it: Problem Statement

If you to add new features to an existing object without having to modify it, then you can opt for the Decorator Pattern.

Terminology

• • Decorator function: The decorator function takes an object, manipulates it using its own object and returns this latter object. For example, in the event of decorating a function:

def decoratorFunction(inputFunction):
def manipulateInputFunction():
capture return value of inputFunction
return manipulatedReturnValueOfInputFunction
return manipulateInputFunction
@decoratorFunction
def functionToBeDecorated():
# body of function
returns an object, say a string
SIGNIFICANCE OF @ NOTATION
Any call to functionToBeDecorated() BECOMES call to decoratorFunction() with functionToBeDecorated as its argument i.e.
functionToBeDecorated() BECOMES decoratorFunction(functionToBeDecorated)()
For example:
stringOne = functionToBeDecorated()
BECOMES
stringOne = decoratorFunction(functionToBeDecorated)()

Pseudo Code
Following is an example of pseudo code that decorates a function. The decorator function decorateMyFunction() takes a function as input and its return value is also a function. Before beginning, it is important to understand that functions, like everything else in Python, is an object.
def decorateMyFunction(originalFunction):
'''Decorates a function by wrapping its return value in a pair of HTML paragraph tags.'''
def addAdditionalText():
obtain string returned by original function
add text to the string
return new string
return addAdditionalText
@decorateMyFunction
def functionToBeDecorated():
'''A simple function that returns a string.'''
# TESTING THE CODE
print( functionToBeDecorated() )

How to implement it
Let's make the output of a function fancier by wrapping its return value with additional text. We will make use of the decorator annotation (@) provided by Python.
def decorateMyFunction(originalFunction):
'''Decorates a function by wrapping its return value in a pair of HTML paragraph tags.'''
def addAdditionalText():
# Obtain string returned by original function
textFromOriginalFunction = originalFunction()
# Adding new functionality to the function being decorated
return "<p>" + textFromOriginalFunction + "</p>"
return addAdditionalText
@decorateMyFunction
def functionToBeDecorated():
'''A simple function that returns a string.'''
return "Hi there!"
print( functionToBeDecorated() )                  # OUTPUT: <p>Hi there!</p>

Walkthrough of implementation

The function declared after the @decorateMyFunction gets passed to the decorateMyFunction as argument.
So, a call to functionToBeDecorated() becomes a call to decorateMyFunction(functionToBeDecorated).
The decorateMyFunction() method returns the function called addAdditionalText. So, the addAdditionalText() function is called.
The addAdditionalText() function captures the return value of the function supplied to it i.e. functionToBeDecorated(), and adds additional text to it.
This manipulated string is returned and is eventually printed.


Related to: Adapter, Composite & Strategy


Creational Patterns
Factory
Abstract Factory
Prototype
Singleton
Builder


Structural Patterns
Decorator
Facade
Proxy
Composite
Adapter
Bridge


Behavioral Patterns
Visitor
Observer
Strategy
Iterator
Mediator
Chain of Responsibility


Architectural Pattern
Model View Controller (MVC)


See also:

Python 101
Object Oriented Python
50+ Handy Standard Library Modules
60+ Handy Third Library Modules
50+ Tips & Tricks for Python Developers
50+ Know-How(s) Every Pythonista Must Know