134 lines
3.8 KiB
Plaintext
134 lines
3.8 KiB
Plaintext
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PEP: 281
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Title: Loop Counter Iteration with range and xrange
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Version: $Revision$
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Last-Modified: $Date$
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Author: magnus@hetland.org (Magnus Lie Hetland)
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Status: Draft
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Type: Standards Track
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Created: 11-Feb-2002
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Python-Version: 2.3
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Abstract
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This PEP describes yet another way of exposing the loop counter in
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for-loops. It basically proposes that the functionality of the
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function indices() from PEP 212 [1] be included in the existing
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functions range() and xrange().
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Motivation
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It is often desirable to loop over the indices of a sequence. PEP
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212 describes several ways of doing this, including adding a
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built-in function called indices, conceptually defined as
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def indices(sequence):
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return range(len(sequence))
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On the assumption that adding functionality to an existing built-in
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function may be less intrusive than adding a new built-in function,
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this PEP proposes adding this functionality to the existing
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functions range() and xrange().
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Specification
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It is proposed that all three arguments to the built-in functions
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range() and xrange() are allowed to be objects with a length
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(i.e. objects implementing the __len__ method). If an argument
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cannot be interpreted as an integer (i.e. it has no __int__
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method), its length will be used instead.
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Examples:
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>>> range(range(10))
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[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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>>> range(range(5), range(10))
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[5, 6, 7, 8, 9]
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>>> range(range(5), range(10), range(2))
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[5, 7, 9]
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>>> list(xrange(range(10)))
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[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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>>> list(xrange(xrange(10)))
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[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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# Number the lines of a file:
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lines = file.readlines()
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for num in range(lines):
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print num, lines[num]
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Alternatives
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A natural alternative to the above specification is allowing
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xrange() to access its arguments in a lazy manner. Thus, instead
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of using their length explicitly, xrange can return one index for
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each element of the stop argument until the end is reached. A
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similar lazy treatment makes little sense for the start and step
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arguments since their length must be calculated before iteration
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can begin. (Actually, the length of the step argument isn't needed
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until the second element is returned.)
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A pseudo-implementation (using only the stop argument, and assuming
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that it is iterable) is:
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def xrange(stop):
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i = 0
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for x in stop:
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yield i
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i += 1
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Testing whether to use int() or lazy iteration could be done by
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checking for an __iter__ attribute. (This example assumes the
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presence of generators, but could easily have been implemented as a
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plain iterator object.)
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It may be questionable whether this feature is truly useful, since
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one would not be able to access the elements of the iterable object
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inside the for loop through indexing.
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Example:
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# Printing the numbers of the lines of a file:
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for num in range(file):
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print num # The line itself is not accessible
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A more controversial alternative (to deal with this) would be to
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let range() behave like the function irange() of PEP 212 when
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supplied with a sequence.
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Example:
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>>> range(5)
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[0, 1, 2, 3, 4]
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>>> range('abcde')
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[(0, 'a'), (1, 'b'), (2, 'c'), (3, 'd'), (4, 'e')]
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Backwards Compatibility
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The proposal could cause backwards incompatibilities if arguments
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are used which implement both __int__ and __len__ (or __iter__ in
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the case of lazy iteration with xrange). The author does not
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believe that this is a significant problem.
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References and Footnotes
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[1] PEP 212, Loop Counter Iteration
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http://www.python.org/peps/pep-0212.html
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Copyright
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This document has been placed in the public domain.
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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fill-column: 70
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End:
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