"""

*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;DR

Adds behaviour to object without affecting its class.

"""

class TextTag:

"""Represents a base text tag"""

def __init__(self, text: str) -> None:

self._text = text

def render(self) -> str:

return self._text

class BoldWrapper(TextTag):

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

def __init__(self, wrapped: TextTag) -> None:

self._wrapped = wrapped

def render(self) -> str:

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

class ItalicWrapper(TextTag):

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

def __init__(self, wrapped: TextTag) -> None:

self._wrapped = wrapped

def render(self) -> str:

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

def main():

"""

>>> simple_hello = TextTag("hello, world!")

>>> special_hello = ItalicWrapper(BoldWrapper(simple_hello))

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

before: hello, world!

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

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

"""

if __name__ == "__main__":

import doctest

doctest.testmod()