Bugs item #1121416, was opened at 2005-02-12 12:18 Message generated for change (Comment added) made by rhettinger You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1121416&group_id=5470
Category: Documentation Group: None Status: Open Resolution: None Priority: 5 Submitted By: Alan (aisaac0) >Assigned to: Raymond Hettinger (rhettinger) Summary: zip incorrectly and incompletely documented Initial Comment: See the zip documentation: http://www.python.org/dev/doc/devel/lib/built-in-funcs.html i. documentation refers to sequences not to iterables ii. The other problem is easier to explain by example. Let it=iter([1,2,3,4]). What is the result of zip(*[it]*2)? The current answer is: [(1,2),(3,4)], but it is impossible to determine this from the docs, which would allow [(1,3),(2,4)] instead (or indeed other possibilities). The example expresses the solution to an actual need, so the behavior should be documented or warned against, I believe. ---------------------------------------------------------------------- >Comment By: Raymond Hettinger (rhettinger) Date: 2005-02-12 14:50 Message: Logged In: YES user_id=80475 The problem with your example does not lie with zip(). Instead, there is a misunderstanding of iter() and how iterators are consumed. Instead of iter(), the correct function is itertools.tee(): >>> zip(*tee([1,2,3,4])) [(1, 1), (2, 2), (3, 3), (4, 4)] Also, stylistically, the zip(*func) approach is too opaque. It is almost always better (at least for other readers and possibly for yourself) to write something more obvious in its intent and operation. List comprehensions and generator expressions are often more clear and easier to write correctly: >>> [(x,x) for x in [1,2,3,4]] [(1, 1), (2, 2), (3, 3), (4, 4)] I do agree that the word sequence should be dropped because it implies that non-sequence iterables are not acceptable as arguments. That's too bad because the word "sequence" seems to help people understand what zip is doing. You're correct that the zip docs do not describe its implementation in such detail as to be able to predict the [(1,2),(3,4)] result. However, that would be an over-specification. That particular result is an implementation specific detail that is subject to change. It probably won't change, but we don't want to encourage people to write code that relies on the specific order of operations within zip(). If someone wants to do something tricky, such as [(1,2),(3,4)], then they are better off writing an explicit loop so that the order of operation is clear both to themselves and to code reviewers. ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1121416&group_id=5470 _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
