python-peps/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)
2005-06-17 14:28:49 -04:00
Status: Rejected
Type: Standards Track
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 [1] be included in the existing
functions range() and xrange().
2005-06-17 00:57:07 -04:00
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.
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.
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