Feature Requests item #1283110, was opened at 2005-09-06 10:37 Message generated for change (Comment added) made by rhettinger You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=355470&aid=1283110&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: Tim Peters (tim_one) Assigned to: Nobody/Anonymous (nobody) Summary: Give __len__() advice for "don't know" Initial Comment: A class may wish to define __len__() despite that it doesn't know how many objects iterating an instance may produce. The docs should give advice for this case. They should also explicitly point out that the result of __len__() is generally taken to be a _hint_ (functions like map(), tuple(), etc., may preallocate space based on __len__()'s result, but adjust to the actual number of objects produced). Tail end of a related thread: <http://mail.zope.org/pipermail/zope3-dev/2005- September/015599.html> ---------------------------------------------------------------------- >Comment By: Raymond Hettinger (rhettinger) Date: 2005-09-07 00:24 Message: Logged In: YES user_id=80475 FWIW, I had explored this topic at some length with Guido's input and arrived at somewhat similar guidance. At any given time, a dynamic object with a __len__() method should report accurately or not at all (raising a TypeError). Accurate means that at that moment, len(d) == len(list(d)). Subsequent mutations may affect the length and the __len__() method is expected to adjust accordingly. For more notes on the subject, see the module docstring for Lib/test/test_iterlen.py In the face of dynamic inputs, consumer functions like map() must regard the reported lengths as estimates. However, that should not lower the standards for objects reporting their length. That number should continue to have some meaning such as the point-in-time invariant described above. ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=355470&aid=1283110&group_id=5470 _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
