Raymond Hettinger wrote: > [Walter Dörwald] >> I'd like to propose the following addition to itertools: A function >> itertools.getitem() which is basically equivalent to the following >> python code: >> >> _default = object() >> >> def getitem(iterable, index, default=_default): >> try: >> return list(iterable)[index] >> except IndexError: >> if default is _default: >> raise >> return default >> >> but without materializing the complete list. Negative indexes are >> supported too (this requires additional temporary storage for abs(index) >> objects). > > Why not use the existing islice() function? > > x = list(islice(iterable, i, i+1)) or default
This doesn't work, because it produces a list >>> list(islice(xrange(10), 2, 3)) or 42 [2] The following would work: x = (list(islice(iterable, i, i+1)) or [default])[0] However islice() doesn't support negative indexes, getitem() does. > Also, as a practical matter, I think it is a bad idea to introduce > __getitem__ style access to itertools because the starting point > moves with each consecutive access: > > # access items 0, 2, 5, 9, 14, 20, ... > for i in range(10): > print getitem(iterable, i) > > Worse, this behavior changes depending on whether the iterable > is re-iterable (a string would yield consecutive items while a > generator would skip around as shown above). islice() has the same "problem": >>> from itertools import * >>> iterable = iter(xrange(100)) >>> for i in range(10): ... print list(islice(iterable, i, i+1)) [0] [2] [5] [9] [14] [20] [27] [35] [44] [54] >>> iterable = xrange(100) >>> for i in range(10): ... print list(islice(iterable, i, i+1)) [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] > Besides being a bug factory, I think the getitem proposal would > tend to steer people down the wrong road, away from more > natural solutions to problems involving iterators. I don't think that (list(islice(iterable, i, i+1)) or [default])[0] is more natural than getitem(iterable, i, default) > A basic step > in learning the language is to differentiate between sequences > and general iterators -- we should not conflate the two. Servus, Walter _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
