A question about Python Classes

Jean-Michel Pichavant jeanmichel at sequans.com
Fri Apr 22 11:47:18 CEST 2011


MRAB wrote:
> On 21/04/2011 18:12, Pascal J. Bourguignon wrote:
>> chad<cdalten at gmail.com>  writes:
>>
>>> Let's say I have the following....
>>>
>>> class BaseHandler:
>>>      def foo(self):
>>>          print "Hello"
>>>
>>> class HomeHandler(BaseHandler):
>>>      pass
>>>
>>>
>>> Then I do the following...
>>>
>>> test = HomeHandler()
>>> test.foo()
>>>
>>> How can HomeHandler call foo() when I never created an instance of
>>> BaseHandler?
>>
>> But you created one!
>>
> No, he didn't, he created an instance of HomeHandler.
I think this is really wrong within the OP question context. This is a 
key concept about OOP and inheritance, any object of class HomeHandler 
is an object of class BaseHandler and also an object of any other base 
class.

However it is true that for convenience, there are some language abuses 
around this terms that we're all using, for instance:
  - class refers to the lowest class of the object (like the python 
__class__ attribute)
  - "instance of" means sometimes "created using the constructor of" 
when stating for example  that you cannot create an instance of a 
virtual class. From a formal OO POV, anyone can create an instance of a 
virtual class, you just need to create an instance of one of its 
subclass. What you cannot, is create a instance of a virtual class using 
the constructor of that virtual class.

Also note that
isinstance(test, BaseHandler)
returns True. And this is no Python trick to trigger some magic, this is 
what OO is about.


JM



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