Steven Bethard <steven.bethard@gmail.com> added the comment: The problem still exists in current trunk: The slicing semantics have been removed from the expressions reference: http://docs.python.org/py3k/reference/expressions.html#slicings The datamodel and types sections still have the same equations: http://docs.python.org/py3k/reference/datamodel.html#types http://docs.python.org/py3k/library/stdtypes.html#typesseq Here's a synopsis of the problem, in code:

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# positive step, works as described i, j, k = 2, 8, 2 range(10)[i:j:k] [2, 4, 6] [i + n * k for n in range(0, (j - i) / k)] [2, 4, 6] [range(10)[i] for i in _] [2, 4, 6]

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# negative step, does not work as described i, j, k = 10, 0, -2 range(10)[i:j:k] [9, 7, 5, 3, 1] [i + n * k for n in range(0, (j - i) / k)] [10, 8, 6, 4, 2] [range(10)[i] for i in _] Traceback (most recent call last): File "<stdin>", line 1, in <module> IndexError: list index out of range

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# actual behavior trims 10 to 9 (is the same as 9:0:-2) # that is, it trims to len(s) - 1, not len(s) range(10)[9:0:-2] [9, 7, 5, 3, 1]

I propose to ignore the definition in the datamodel section, since it's talking about extended slicing in general, and doesn't mention anything about negative indices. I propose the attached patch to fix the definition in the types section, which simply changes the statement: If i or j is greater than len(s), use len(s) to say: If *i* or *j* is greater than len(s), use len(s) if *k* is positive, or len(s) - 1 if *k* is negative. ---------- keywords: +patch versions: +Python 2.7, Python 3.2, Python 3.3 -Python 2.6, Python 3.0 Added file: http://bugs.python.org/file21428/extended_slicing_docs.diff _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue1446619> _______________________________________