Thanks Calvin, Could you please expand on your thoughts about possible confusion? That is, how do you see a programmer becoming confused if this option were added to the syntax.
On Mon, Jan 19, 2009 at 11:29 AM, Calvin Spealman <ironfro...@gmail.com> wrote: > I am really unconvinced of the utility of this proposal and quite > convinced of the confusing factor it may well add to the current > syntax. I would like to see more applicable examples. It would replace > uses of takewhile, but that isn't a really often used function. So, is > there any evidence to support that making this a new syntax would find > so many more uses of the construct to be worth it? I believe not. > > On Mon, Jan 19, 2009 at 10:10 AM, Gerald Britton > <gerald.brit...@gmail.com> wrote: >> Please find below PEP 3142: Add a "while" clause to generator >> expressions. I'm looking for feedback and discussion. >> >> >> PEP: 3142 >> Title: Add a "while" clause to generator expressions >> Version: $Revision: 68715 $ >> Last-Modified: $Date: 2009-01-18 11:28:20 +0100 (So, 18. Jan 2009) $ >> Author: Gerald Britton <gerald.brit...@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. Of course, this >> could also be achieved like this: >> >> p = (p for p in (p for p in primes() if p > 100) while p < 1000) >> >> which is syntactically simpler. Some may also ask if it is possible >> to cover dropwhile() functionality in a similar way. I initially thought >> of: >> >> p = (p for p in primes() not while p < 100) >> >> but I am not sure that I like it since it uses "not" in a non-pythonic >> fashion, I think. >> >> Adding a "while" clause to generator expressions maintains the >> compact form while adding a useful facility for short-circuiting >> the expression. >> >> Implementation: >> >> I am willing to assist in the implementation of this feature, although I >> have >> not contributed to Python thus far and would definitely need mentoring. (At >> this point I am not quite sure where to begin.) Presently though, I would >> find it challenging to fit this work into my existing workload. >> >> >> 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: >> >> _______________________________________________ >> Python-Dev mailing list >> Python-Dev@python.org >> http://mail.python.org/mailman/listinfo/python-dev >> Unsubscribe: >> http://mail.python.org/mailman/options/python-dev/ironfroggy%40gmail.com >> >> > > > > -- > Read my blog! I depend on your acceptance of my opinion! I am interesting! > http://techblog.ironfroggy.com/ > Follow me if you're into that sort of thing: http://www.twitter.com/ironfroggy > _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
