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#!/usr/bin/env python

# -*- coding: utf-8 -*-

"""

*What is this pattern about?

The Decorator pattern is used to dynamically add a new feature to an

object without changing its implementation. It differs from

inheritance because the new feature is added only to that particular

object, not to the entire subclass.

*What does this example do?

This example shows a way to add formatting options (boldface and

italic) to a text by appending the corresponding tags (<b> and

<i>). Also, we can see that decorators can be applied one after the other,

since the original text is passed to the bold wrapper, which in turn

is passed to the italic wrapper.

*Where is the pattern used practically?

The Grok framework uses decorators to add functionalities to methods,

like permissions or subscription to an event:

http://grok.zope.org/doc/current/reference/decorators.html

*References:

https://sourcemaking.com/design_patterns/decorator

*TL;DR80

Adds behaviour to object without affecting its class.

"""

from __future__ import print_function

class TextTag(object):

"""Represents a base text tag"""

def __init__(self, text):

self._text = text

def render(self):

return self._text

class BoldWrapper(TextTag):

"""Wraps a tag in <b>"""

def __init__(self, wrapped):

self._wrapped = wrapped

def render(self):

return "<b>{}</b>".format(self._wrapped.render())

class ItalicWrapper(TextTag):

"""Wraps a tag in <i>"""

def __init__(self, wrapped):

self._wrapped = wrapped

def render(self):

return "<i>{}</i>".format(self._wrapped.render())

if __name__ == '__main__':

simple_hello = TextTag("hello, world!")

special_hello = ItalicWrapper(BoldWrapper(simple_hello))

print("before:", simple_hello.render())

print("after:", special_hello.render())

### OUTPUT ###

# before: hello, world!

# after: <i><b>hello, world!</b></i>