[Python-checkins] r68715 - peps/trunk/pep-3142.txt
georg.brandl
python-checkins at python.org
Sun Jan 18 11:28:20 CET 2009
Author: georg.brandl
Date: Sun Jan 18 11:28:20 2009
New Revision: 68715
Log:
Add PEP 3142.
Added:
peps/trunk/pep-3142.txt (contents, props changed)
Added: peps/trunk/pep-3142.txt
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+++ peps/trunk/pep-3142.txt Sun Jan 18 11:28:20 2009
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+PEP: 3142
+Title: Add a "while" clause to generator expressions
+Version: $Revision$
+Last-Modified: $Date$
+Author: Gerald Britton <gerald.britton at gmail.com>
+Status: Draft
+Type: Standards Track
+Content-Type: text/plain
+Created: 12-Jan-2009
+Python-Version: 3.0
+Post-History:
+
+
+Abstract
+
+ This PEP proposes an enhancement to generator expressions, adding a
+ "while" clause to complement the existing "if" clause.
+
+
+Rationale
+
+ A generator expression (PEP 289 [1]) is a concise method to serve
+ dynamically-generated objects to list comprehensions (PEP 202 [2]).
+ Current generator expressions allow for an "if" clause to filter
+ the objects that are returned to those meeting some set of
+ criteria. However, since the "if" clause is evaluated for every
+ object that may be returned, in some cases it is possible that all
+ objects would be rejected after a certain point. For example:
+
+ g = (n for n in range(100) if n*n < 50)
+
+ which is equivalent to the using a generator function
+ (PEP 255 [3]):
+
+ def __gen(exp):
+ for n in exp:
+ if n*n < 50:
+ yield n
+ g = __gen(iter(range(10)))
+
+ would yield 0, 1, 2, 3, 4, 5, 6 and 7, but would also consider
+ the numbers from 8 to 99 and reject them all since n*n >= 50 for
+ numbers in that range. Allowing for a "while" clause would allow
+ the redundant tests to be short-circuited:
+
+ g = (n for n in range(100) while n*n < 50)
+
+ would also yield 0, 1, 2, 3, 4, 5, 6 and 7, but would stop at 8
+ since the condition (n*n < 50) is no longer true. This would be
+ equivalent to the generator function:
+
+ def __gen(exp):
+ for n in exp:
+ if n*n < 50:
+ yield n
+ else:
+ break
+ g = __gen(iter(range(100)))
+
+ Currently, in order to achieve the same result, one would need to
+ either write a generator function such as the one above or use the
+ takewhile function from itertools:
+
+ from itertools import takewhile
+ g = takewhile(lambda n: n*n < 50, range(100))
+
+ The takewhile code achieves the same result as the proposed syntax,
+ albeit in a longer (some would say "less-elegant") fashion. Also,
+ the takewhile version requires an extra function call (the lambda
+ in the example above) with the associated performance penalty.
+ A simple test shows that:
+
+ for n in (n for n in range(100) if 1): pass
+
+ performs about 10% better than:
+
+ for n in takewhile(lambda n: 1, range(100)): pass
+
+ though they achieve similar results. (The first example uses a
+ generator; takewhile is an iterator). If similarly implemented,
+ a "while" clause should perform about the same as the "if" clause
+ does today.
+
+ The reader may ask if the "if" and "while" clauses should be
+ mutually exclusive. There are good examples that show that there
+ are times when both may be used to good advantage. For example:
+
+ p = (p for p in primes() if p > 100 while p < 1000)
+
+ should return prime numbers found between 100 and 1000, assuming
+ I have a primes() generator that yields prime numbers.
+
+ Adding a "while" clause to generator expressions maintains the
+ compact form while adding a useful facility for short-circuiting
+ the expression.
+
+
+Acknowledgements
+
+ Raymond Hettinger first proposed the concept of generator
+ expressions in January 2002.
+
+
+References
+
+ [1] PEP 289: Generator Expressions
+ http://www.python.org/dev/peps/pep-0289/
+
+ [2] PEP 202: List Comprehensions
+ http://www.python.org/dev/peps/pep-0202/
+
+ [3] PEP 255: Simple Generators
+ http://www.python.org/dev/peps/pep-0255/
+
+
+Copyright
+
+ This document has been placed in the public domain.
+
+
+Local Variables:
+mode: indented-text
+indent-tabs-mode: nil
+sentence-end-double-space: t
+fill-column: 70
+coding: utf-8
+End:
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