A question about Python Classes

[Steven D'Aprano]

On Sat, 23 Apr 2011 13:30:02 -0700, chad wrote:

> On Apr 22, 12:47 pm, Carl Banks <pavlovevide... at gmail.com> wrote:
>> On Thursday, April 21, 2011 11:00:08 AM UTC-7, MRAB wrote:
>> > On 21/04/2011 18:12, Pascal J. Bourguignon wrote:
>> > > chad<cda... 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.
>>
>> > > test is an instance of HomeHandler, which is a subclass of
>> > > BaseHandler, so test is also an instance of BaseHandler.
>>
>> > test isn't really an instance of BaseHandler, it's an instance of
>> > HomeHandler, which is a subclass of BaseHandler.
>>
>> I'm going to vote that this is incorrect usage.  An instance of
>> HomeHandler is also an instance of BaseHandler, and it is incorrect to
>> say it is not.  The call to HomeHandler does create an instance of
>> BaseHandler.
>>
>>
> What do you mean by the "call to HomeHandler"?  Don't I call HomeHandler
> after I create an instance of BaseHandler?

Not directly/explicitly.

The process you do is:

(1) Create the class BaseHandler.
(2) Create the class HomeHandler, which is a subclass of BaseHandler.
(3) Call HomeHandler.

At no point do you create a *direct* instance of BaseHandler (e.g. by 
calling BaseHandler). However the instance of HomeHandler is ALSO an 
instance of BaseHandler by virtue of the is-subclass relationship:

>>> issubclass(HomeHandler, BaseHandler)
True
>>> isinstance(test, HomeHandler)
True
>>> isinstance(test, BaseHandler)
True


If you *had* created a direct instance of BaseHandler by calling 
BaseHandler:

>>> base = BaseHandler()
>>> isinstance(base, BaseHandler)
True
>>> isinstance(base, HomeHandler):
False


It's like this... "Toyota Prius" is a subclass of "Motor Vehicle", so 
each and every Prius car is also a motor vehicle. But not every motor 
vehicle is a Toyota Prius.


-- 
Steven