Difference between type and class

Thomas Troeger thomas.troeger.ext at siemens.com
Thu Jul 31 13:32:39 CEST 2008

> 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):

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).


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