The following trace shows what I mean:
>>> help(str.__getslice__)
Help on wrapper_descriptor:

__getslice__(...)
    x.__getslice__(i, j) <==> x[i:j]
>>> 'ab'[-1]
'b'
>>> 'ab'.__getitem__(-1)
'b'
>>> 'ab'[-2:-1]
'a'
>>> 'ab'.__getslice__(-2,-1)
''

that is, __getslice__ does not add the length of the
sequence to its arguments when they are negative as
indexing with [:] does.
This contradicts the docstring which says __getslice__
is equivalent to indexing.
The same behavior applies to lists.

Note that __getitem__ does it right.

This bug may be linked to bug 473985 ('str, __getitem__
and slices') but should be much easier to fix.

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Tim,  I attached a tiny patch which fixes the behavior for 
strings.  I've also tried out the same change in listobject.c 
and unicodeobject.c.  Is this the right this to do or are 
there more subtleties to this than meet the eye?

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This should NOT be fixed this way, Raymond!

The docs are (subtly) wrong. For historical reasons, 
__getslice__ expects the Python VM to do the adjustment of 
negative indices. If you did another adjustment inside getslice, 
certain indices would start behaving differently (e.g. s[:-5] 
where len(s)==3).

Maybe the docstring should be fixed.

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If the docstring fix is still the best way to fix this I am willing to make the 
change.

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Yes, please fix the docstring.

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Fixed as revision 2.232 on HEAD (after getting it right; bah) and as 
2.126.4.35 on release22-maint.