Maybe instead of referring to mappings it should say "The primary must implement __getitem__(), which is called with a value that is constructed from the slice list, as follows."
In the section you mention, 'mapping' is equivalent to 'has __getitem__ defined', and I'd be surprised if it's an isolated usage. Python actually has trouble distinguishing between sequences and mappings, as anyone who as tried to use the 'isMapping' API would know (the isMapping API uses much the same definition as the reference manual does - so all sequences show up as mappings, as they map indices and slices to objects).
Section 3 of the reference manual is actually more use for anyone developing custom types that override special methods. E.g. proper handling of slice objects is described here under container emulation:
http://www.python.org/dev/doc/devel/ref/sequence-types.html
Regards, Nick.
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