decorator.py

#!/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>