•   Abstract Base Classes don't require more syntax.
•   Zope and Twisted can't live without interfaces, and they have created their own implementation.
•   The interfaces and type checking are interconnected.
•   Interfaces make much more sense if you can also declare the argument types of the methods.
•   Phillip Eby's proposal of  an interface-free alternative is too complex to be adopted as standard Python mechanism.
•   Ping suggests that there is a need to specify some semantics in interfaces.
•   Python has a strong tradition that subclasses may redefine methods with a different signature.
•   Its not decided that Python 3.0 will have interfaces,  will look at some of the standard interfaces the language would provide for various common protocols like sequence, mapping and file.
•   The File Interface : they don't require genericity to fully specify the interface.
•   The Sequence Interface : they are both useful and easy to implement, even if there are some thorny issues left.

Reference:
>> Interfaces or Abstract Base Classes?

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