Bugs item #1265100, was opened at 2005-08-20 15:30 Message generated for change (Settings changed) made by bediviere You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1265100&group_id=5470
Please note that this message will contain a full copy of the comment thread, including the initial issue submission, for this request, not just the latest update. >Category: Documentation Group: None Status: Open Resolution: None Priority: 5 Submitted By: Steven Bethard (bediviere) Assigned to: Nobody/Anonymous (nobody) Summary: sequence slicing documentation incomplete Initial Comment: Here's what points (3) and (5) of the Sequence Types[1] documentation say now: """ (3) If i or j is negative, the index is relative to the end of the string: len(s) + i or len(s) + j is substituted. But note that -0 is still 0. ... (5) The slice of s from i to j with step k is defined as the sequence of items with index x = i + n*k such that $0 \leq n < \frac{j-i}{k}$. In other words, the indices are i, i+k, i+2*k, i+3*k and so on, stopping when j is reached (but never including j). If i or j is greater than len(s), use len(s). If i or j are omitted then they become ``end'' values (which end depends on the sign of k). Note, k cannot be zero. """ I'd like to replace that second-to-last sentence in point (5), which uses the vague word "end'' in its description, with something more explicit. I'd like it to read something like: """ If k is positive and i or j is omitted, they will be replaced with 0 and len(s) respectively. If k is negative and i or j is omitted, they will be replaced with -1 and -len(s)-1 respectively. Note that these replacements happen before the rule from point (3) is applied. """ I'd also like to put an example with point (5) to show this behavior in action. Something like: """ So for example:: >>> seq = 'abcde' >>> len(seq) 5 >>> 'edc' == seq[:1:-1] == seq[-1:1:-1] True >>> 'ca' == seq[2::-2] == seq[2:-5-1:-2] True Note however that manually applying the rule from point (3) (adding len(s) to any i or j that is negative) works for i, but does not work for j:: >>> seq[5-1:1:-1] 'edc' >>> seq[2:-1:-2] '' This is because Python sees the -1 in the j position and applies the rule from point (3) again. """ One of the motivations for this patch is to be able to explain the difference in behavior between sequence slicing and slice.indices(), e.g. >>> seq[::-2] 'eca' >>> slice(None, None, -2).indices(5) (4, -1, -2) >>> seq[4:-1:-2] '' Right now, I think the documentation is too vague to be able to make it clear why slices behave differently. If this patch is accepted, I'll work on a patch for slice objects that explains the differences. [1] http://docs.python.org/lib/typesseq.html ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1265100&group_id=5470 _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com