inheritance - Subclassing in python of instantiated superclass -

is possible subclass in python using instantiated superclass?

i don't know how frame question let me give example. suppose have class rectangle , want build class coloredrectangle. don't want each coloredrectangle of same dimensions own new rectangle. when initiate coloredrectangle pass instantiated rectangle.

ie., want

r = rectangle([1,2]) r_red = coloredrectangle(r, "red") r_blue = coloredrectangle(r, "blue") 

but r_red , r_blue should able rectangle methods , attributes. example suppose rectangle had area() attribute.

r.area 2 r_red.area 2 

r_red , r_blue should "point" same rectangle. know writing:

 class coloredrectangle(rectangle):        def __init__(self, rectangle, color):           self.color = color           self.rectangle = rectangle 

but i'd have write

  r_red.rectangle.area 

which ugly.

inheritance

inheritance such nice thing in python, , don't think have resort getattr hacks, if want those, scroll down.

you can force class dictionary refer object:

class rectangle(object):     def __init__(self, width, height):         self.width = width         self.height = height     def area(self):         return self.width * self.height   class coloredrectangle(rectangle):     def __init__(self, rect, color):         self.__dict__ = rect.__dict__         self.color = color   rect = rectangle(3, 5) crect = coloredrectangle(rect, color="blue") print crect.width, crect.height, crect.color #3 5 blue 

these 2 refer same rectangle object:

crect.width=10 print rect.width, rect.height #10 5 

this exellent talk on metaprogramming, , while it's title implies python3 lot of applies python 2.x: david beazley - python3 metaprogramming

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getattr hacking

if reason however, want have multiple coloredrectangle refer same base rectangle these conflict each other:

eve = rectangle(3, 5) kain = coloredrectangle(eve, color="blue") abel = coloredrectangle(eve, color="red") print eve.color, kain.color, abel.color #red red red 

if you'd different "proxy objects", can attributes base rectangle not interfere each other, have resort getattr hacking, fun too:

class coloredrectangle(rectangle):     def __init__(self, rect, color):         self.rect = rect         self.color = color     def __getattr__(self,attr):         return getattr(self.rect,attr) eve = rectangle(3, 5) 

this avoid interference:

kain = coloredrectangle(eve, color="blue") abel = coloredrectangle(eve, color="red") print kain.color, abel.color #blue red 

about __getattr__ versus __getattribute__:

a key difference between getattr , getattribute getattr invoked if attribute wasn't found usual ways. it's implementing fallback missing attributes, , 1 of 2 want. source

because non found attributes handled __getattr__ can partially update proxies, might confusing:

kain.width=10 print eve.area(), kain.area(), abel.area() # 15 50 15 

to avoid can override __setattr__:

def __setattr__(self, attr, value):     if attr == "color":         return super(coloredrectangle,self).setattr(attr,value)     raise yourfavoriteexception