A question about Python Classes

Terry Reedy tjreedy at udel.edu
Thu Apr 21 19:39:17 CEST 2011


On 4/21/2011 11:43 AM, chad wrote:
> 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?

When you ask for an attribute of an instance of a class, the attribute 
lookup first looks at the instance; if not there, then the class; if not 
there, then superclass(es); and so on back to class 'object'.

 >>> class C(): pass

 >>> c=C()
 >>> c.__hash__
<method-wrapper '__hash__' of C object at 0x00FCB5D0>

# how does this happen when C has no __hash__ method?

 >>> C.__hash__
<slot wrapper '__hash__' of 'object' objects>

# C inherits __hash__ and other special methods from 'object'

 >>> hash(c)
1035101

# uses the default, inherited method.

Most syntactic operations and builtins are ultimately converted to a 
special method call, often inherited like this. In fact, c.x is 
converted to object.__getattribute__(c, 'x').

 >>> object.__getattribute__(c, '__hash__')
<method-wrapper '__hash__' of C object at 0x00FCB5D0>

You do need to understand inheritance. On the other hand, do not worry 
about behind-the-scenes implementation details like 'method_wrapper' and 
'slot_wrapper' classes, which may be CPython specific.

-- 
Terry Jan Reedy




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