PEP: 281 Title: Loop Counter Iteration with range and xrange Author: Magnus Lie Hetland Status: Rejected Type: Standards Track Content-Type: text/x-rst Created: 11-Feb-2002 Python-Version: 2.3 Post-History: Abstract ======== This PEP describes yet another way of exposing the loop counter in for-loops. It basically proposes that the functionality of the function ``indices()`` from :pep:`212` be included in the existing functions ``range()`` and ``xrange()``. Pronouncement ============= In commenting on :pep:`279`'s ``enumerate()`` function, this PEP's author offered, "I'm quite happy to have it make :pep:`281` obsolete." Subsequently, :pep:`279` was accepted into Python 2.3. On 17 June 2005, the BDFL concurred with it being obsolete and hereby rejected the PEP. For the record, he found some of the examples to somewhat jarring in appearance:: >>> range(range(5), range(10), range(2)) [5, 7, 9] Motivation ========== It is often desirable to loop over the indices of a sequence. PEP 212 describes several ways of doing this, including adding a built-in function called indices, conceptually defined as:: def indices(sequence): return range(len(sequence)) On the assumption that adding functionality to an existing built-in function may be less intrusive than adding a new built-in function, this PEP proposes adding this functionality to the existing functions ``range()`` and ``xrange()``. Specification ============= It is proposed that all three arguments to the built-in functions ``range()`` and ``xrange()`` are allowed to be objects with a length (i.e. objects implementing the ``__len__`` method). If an argument cannot be interpreted as an integer (i.e. it has no ``__int__`` method), its length will be used instead. Examples:: >>> range(range(10)) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> range(range(5), range(10)) [5, 6, 7, 8, 9] >>> range(range(5), range(10), range(2)) [5, 7, 9] >>> list(xrange(range(10))) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> list(xrange(xrange(10))) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] # Number the lines of a file: lines = file.readlines() for num in range(lines): print num, lines[num] Alternatives ============ A natural alternative to the above specification is allowing ``xrange()`` to access its arguments in a lazy manner. Thus, instead of using their length explicitly, ``xrange`` can return one index for each element of the stop argument until the end is reached. A similar lazy treatment makes little sense for the start and step arguments since their length must be calculated before iteration can begin. (Actually, the length of the step argument isn't needed until the second element is returned.) A pseudo-implementation (using only the stop argument, and assuming that it is iterable) is:: def xrange(stop): i = 0 for x in stop: yield i i += 1 Testing whether to use ``int()`` or lazy iteration could be done by checking for an ``__iter__`` attribute. (This example assumes the presence of generators, but could easily have been implemented as a plain iterator object.) It may be questionable whether this feature is truly useful, since one would not be able to access the elements of the iterable object inside the for loop through indexing. Example:: # Printing the numbers of the lines of a file: for num in range(file): print num # The line itself is not accessible A more controversial alternative (to deal with this) would be to let ``range()`` behave like the function ``irange()`` of :pep:`212` when supplied with a sequence. Example:: >>> range(5) [0, 1, 2, 3, 4] >>> range('abcde') [(0, 'a'), (1, 'b'), (2, 'c'), (3, 'd'), (4, 'e')] Backwards Compatibility ======================= The proposal could cause backwards incompatibilities if arguments are used which implement both ``__int__`` and ``__len__`` (or ``__iter__`` in the case of lazy iteration with ``xrange``). The author does not believe that this is a significant problem. Copyright ========= This document has been placed in the public domain.