Python-Design-Patterns decorator
Table of Contents
decorator Model
Python-Design-Patterns decorator
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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>
decorator.py
decorator Test
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#!/usr/bin/env python
# -*- coding: utf-8 -*-
import unittest
from structural.decorator import TextTag, BoldWrapper, ItalicWrapper
class TestTextWrapping(unittest.TestCase):
def setUp(self):
self.raw_string = TextTag('raw but not cruel')
def test_italic(self):
self.assertEqual(ItalicWrapper(self.raw_string).render(), '<i>raw but not cruel</i>')
def test_bold(self):
self.assertEqual(BoldWrapper(self.raw_string).render(), '<b>raw but not cruel</b>')
def test_mixed_bold_and_italic(self):
self.assertEqual(BoldWrapper(ItalicWrapper(self.raw_string)).render(), '<b><i>raw but not cruel</i></b>')
test_decorator.py
