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
_______________________________________________
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
