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I don't know which Python sources Raymond has been reading,
but in the sources I've got in front of me, there are
special cases for strings and tuples, and these *don't* use
iter(). It so happens that the tuple special-case calls
PyTuple_GetItem(), which doesn't call your __getitem__,
while the string special-case calls the sq_item slot
function, which (in your case) will be a wrapper that calls
your __getitem__.

A minimal fix would be to only call filtertuple for strict
tuples -- although this changes the output type, but I don't
think one should count on filter() of a tuple subclass
returning a tuple (and it can't be made to return an
instance of the subclass either -- we don't know the
constructor signature).

Similar fixes probably need to be made to map() and maybe
reduce().