A question about Python Classes

Thu Apr 21 21:53:47 EDT 2011

On 2011年4月21日 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