A question about Python Classes

Steven D'Aprano steve+comp.lang.python at pearwood.info
Fri Apr 22 03:53:47 CEST 2011


On Thu, 21 Apr 2011 19:00:08 +0100, MRAB wrote:

>>> 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.

Which *also* makes it an instance of BaseHandler. You are a human being, 
which also makes you a mammal. It would be *wrong* to say that you're not 
a mammal, just because you're a human being.

But to answer the Original Poster's question... you don't need a formal 
BaseHandler instance because that's how inheritance is designed to work. 
Each class knows its own parent classes, and when you call test.foo(), 
Python walks the chain of:

instance
instance's class
each of the parent class(es) (if any)

looking for a match for foo, and then calls it appropriately. This is 
called inheritance: HomeHandler inherits behaviour from BaseHandler.

(The details are a little more complex than the sketch above, but broadly 
equivalent.)



-- 
Steven



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