How to extend a class in python?

Use:

import colorclass Color(color.Color):    ...

If this were Python 2.x, you would also want to derive color.Color from object, to make it a new-style class:

class Color(object):    ...

This is not necessary in Python 3.x.

class MyParent:    def sayHi():        print('Mamma says hi')

from path.to.MyParent import MyParentclass ChildClass(MyParent):    pass

An instance of ChildClass will then inherit the sayHi() method.

Another way to extend (specifically meaning, add new methods, not change existing ones) classes, even built-in ones, is to use a preprocessor that adds the ability to extend out of/above the scope of Python itself, converting the extension to normal Python syntax before Python actually gets to see it.

I've done this to extend Python 2's str() class, for instance. str() is a particularly interesting target because of the implicit linkage to quoted data such as 'this' and 'that'.

Here's some extending code, where the only added non-Python syntax is the extend:testDottedQuad bit:

extend:testDottedQuaddef testDottedQuad(strObject):    if not isinstance(strObject, basestring): return False    listStrings = strObject.split('.')    if len(listStrings) != 4: return False    for strNum in listStrings:        try:    val = int(strNum)        except: return False        if val < 0: return False        if val > 255: return False    return True

After which I can write in the code fed to the preprocessor:

if '192.168.1.100'.testDottedQuad():    doSomething()dq = '216.126.621.5'if not dq.testDottedQuad():    throwWarning();dqt = ''.join(['127','.','0','.','0','.','1']).testDottedQuad()if dqt:    print 'well, that was fun'

The preprocessor eats that, spits out normal Python without monkeypatching, and Python does what I intended it to do.

Just as a c preprocessor adds functionality to c, so too can a Python preprocessor add functionality to Python.

My preprocessor implementation is too large for a stack overflow answer, but for those who might be interested, it is here on GitHub.