OK, I still don't like the general idea, but I have a suggestion for what I think is a more favorable syntax, anyway. Basically, adding the allowance of an 'else break' when there is an if clause in a generator expression or list comprehension. I still dont think it should be done, but it is a more consistent syntax. It has the obvious problem of looking like it might allow an alternative expression, like the if-expression.
prime = (p for p in sieve() if p < 1000 else break) On Mon, Jan 19, 2009 at 11:47 AM, Calvin Spealman <ironfro...@gmail.com> wrote: > On Mon, Jan 19, 2009 at 11:41 AM, Gerald Britton > <gerald.brit...@gmail.com> wrote: >> 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. > > I think that the difference between these two lines is too easy to > miss among other code: > > prime = (p for p in sieve() while p < 1000) > > and: > > prime = (p for p in sieve() if p < 1000) > > The very distinctly different behavior is two important to look so > similar at a glance. And, again, I just don't see takewhile being used > all that often. Not nearly often enough to warrant replacing it with a > special syntax! Only 178 results from Google CodeSearch, while chain, > groupby, and repeat get 4000, 3000, and 1000 respectively. Should > those be given their own 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 >>> >> > > > > -- > 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 > -- 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
