Difference between type and class

Nikolaus Rath Nikolaus at rath.org
Thu Jul 31 14:27:25 CEST 2008


Thomas Troeger <thomas.troeger.ext at siemens.com> writes:
>> Can someone explain to me the difference between a type and a class?
>
> If your confusion is of a more general nature I suggest reading the
> introduction of `Design Patterns' (ISBN-10: 0201633612), under
> Specifying Object Interfaces'.
>
> In short: A type denotes a certain interface, i.e. a set of
> signatures, whereas a class tells us how an object is implemented
> (like a blueprint). A class can have many types if it implements all
> their interfaces, and different classes can have the same type if they
> share a common interface. The following example should clarify
> matters:
>
> class A:
> 	def bar(self):
> 		print "A"
>
> class B:
> 	def bar(self):
> 		print "B"
>
> class C:
> 	def bla(self):
> 		print "C"
>
> def foo(x):
> 	x.bar()
>
> you can call foo with instances of both A and B, because both classes
> share a common type, namely the type that has a `bar' method), but not
> with an instance of C because it has no method `bar'. Btw, this
> example shows the use of duck typing
> (http://en.wikipedia.org/wiki/Duck_typing).

That would imply that I cannot create instances of a type, only of
a class that implements the type, wouldn't it?

But Python denotes 'int' as a type *and* I can instantiate it.



Still confused,

   -Nikolaus

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  By definition, there are already enough people to do that.«
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