PEP 281, Loop Counter Iteration with range and xrange, Magnus Lie

Hetland

pep-0281.txt

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+PEP: 281
+Title: Loop Counter Iteration with range and xrange
+Version: $Revision$
+Last-Modified: $Date$
+Author: magnus@hetland.org (Magnus Lie Hetland)
+Status: Draft
+Type: Standards Track
+Created: 11-Feb-2002
+Python-Version: 2.3
+
+
+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 [1] be included in the existing
+   functions range() and xrange().
+
+
+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.
+
+
+References and Footnotes
+
+   [1] PEP 212, Loop Counter Iteration
+       http://www.python.org/peps/pep-0212.html
+
+
+Copyright
+
+    This document has been placed in the public domain.
+
+
+
+Local Variables:
+mode: indented-text
+indent-tabs-mode: nil
+fill-column: 70
+End: