What is the difference between Python decorators and the decorator pattern?

Question

What is the difference between “Python decorators” and the “decorator pattern”?

When should I use Python decorators, and when should I use the decorator pattern?

Answer 1

Decorator Pattern - In object-oriented programming, the decorator pattern is a design pattern that allows behaviour to be added to an existing object dynamically. The decorator pattern can be used to extend (decorate) the functionality of a certain object at run-time, independently of other instances of the same class, provided some groundwork is done at design time.

Decorators in Python - Despite the name, Python decorators are not an implementation of the decorator pattern. The decorator pattern is a design pattern used in statically typed object-oriented programming languages to allow functionality to be added to objects at run time; Python decorators add functionality to functions and methods at definition time, and thus are a higher-level construct than decorator-pattern classes.

The decorator pattern itself is trivially implementable in Python, because the language is duck typed, and so is not usually considered as such. So in Python a decorator is any callable Python object that is used to modify a function, method or class definition.

I hope I made the difference clear. Just in case you did not completely understand, please go through these links. You will come out more than clear at the end of it -

• How to make a chain of function decorators?

• Implementing the decorator pattern in Python

• What is the difference between using decorators and extending a sub class by inheritance?

• Python Class Decorator

• PyWiki - Python Decorators - A detailed discourse

• Python Decorators Made Easy

• Source 1 & source 2

Answer 2

The difference is this:

(a) Python decorators are tied to an existing method and change the behavior of that method. Example:

@modifyBehavior
def original(myString):
    print myString

The behavior of original is overwritten. You can't use this to add a new functionality.

(b) Decorator pattern is about polymorphism. In your sample code above, the behavior of Decorator.something_useful is overwritten. The original method is lost. It's not really decorator pattern. You should be looking to enhance or add functionality, not replace a method. You should ensure that a.something_useful(string) returns the same thing as b.something_useful(string). In fact, in decorator pattern you would typically replace the original object. Here is what I mean:

class Class(object):
    def __init__(self):
        pass
    def something_useful(self, string):
        return string

class Decorator(object):
    def __init__(self, wrapped):
        self._wrapped = wrapped
    def withUnderscores(self, string):
        return '_'.join(string.split(' '))
    def __getattr__(self, name):
        return getattr(self._wrapped, name)


if __name__ == '__main__':
    string = 'Lorem ipsum dolor sit amet.'
    obj = Class()
    print('Original: ', obj.something_useful(string))
    #This has no underscore function.  Use decorator to add.
    obj = Decorator(obj)
    print('Replaced spaces: ', obj.withUnderscores(string))
    print('Original still works: ', obj.something_useful(string))

You can have several decorators to add functionality. This allows you to add only what you need when you need it. More reading: GoF