In python how can you extend a class? For example if I have
color.py
class Color:
def __init__(self, color):
self.color = color
def getcolor(self):
return self.color
color_extended.py
import Color
class Color:
def getcolor(self):
return self.color + " extended!"
But this doesn't work...
I expect that if I work in color_extended.py
, then when I make a color object and use the getcolor
function then it will return the object with the string " extended!" in the end. Also it should have gotton the init from the import.
Assume python 3.1
Thanks
class MyParent:
def sayHi():
print('Mamma says hi')
from path.to.MyParent import MyParent
class 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:testDottedQuad
def 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.
Source: Stackoverflow.com