Consider the following example:

class A:
     def __getitem__(self, index):
         return True

If you invoke A()[-1], everything is fine. However, if you invoke A()[-1:2], you get an "AttributeError: A instance has no attribute '__len__'".

Moreover, if you define __len__ for your class, you will discover that __getitem__ will act "smart" and modify slice you are passing into the function. Check this out:

class A:
    def __getitem__(self, index):
        return index.start
    def __len__(self):
        return 10

Now A()[-1:10] outputs "9". The same kind of argument-mangling happens within __setitem__.

This is completely unintuitive and contrary to what I read in the docs (https://docs.python.org/2/reference/datamodel.html#object.__getitem__):
"Note that the special interpretation of negative indexes (if the class wishes to emulate a sequence type) is up to the __getitem__() method.".

Especially intuitive is the fact that if you do A()[slice(-1,10)] or A().__getitem__(slice(-1,10)), no special treatment is done for the -1, everything works fine and the __len__ method is not invoked.

As far as I understand, the root cause is the behaviour of STORE_SLICE+3 command, which tries to be too smart.

I have discovered this within code where slice indexing was used to insert arbitrary intervals into a data structure (hence using negative numbers would be totally fine), and obviuosly such behaviour broke the whole idea, albeit it was nontrivial to debug and discover.

This does not seem to be a problem for Python 3.3, however I believe fixing this in Python 2.7 is important as well.