Yes of course. However I am advocating adding this to the syntax rather than using the takewhile function, thus making it part of the generator expression proper and avoiding the function call and module include. From the little I know about the implementation of the corresponding "if" clause, this should be relatively easy. On 1/10/09, Mathias Panzenböck wrote:

Mathias Panzenböck schrieb:

...

Gerald Britton schrieb:

...

I've been using Python generators for a while now. e.g.

a=(i for i in range(10))

a.next() a.next() ...etc.

I also find the "if" clause handy:

a = (i for i in range(10) if i%2==0)

(I know that range(0,12,2) will do the same thing, but it's the idea I like, especially for more complex predicates.)

I would like to know if anyone has thought of adding a "while" clause as well, like this:

a = (i for i in range(100) while i <=50)

from itertools takewhile a = takewhile(lambda i: i <= 50, xrange(100))

typo: from itertools import takewhile a = takewhile(lambda i: i <= 50, xrange(100))

-- Sent from my mobile device

I guess I don't think it would be confusing. On the contrary, I believe that the expressions would read naturally and be a nice simplification. Of course it won't work just like "if" but that is just the point! I can (and do) accomplish the same thing with "takewhile", but if the same thing can be sone with a little addition to the generator expression, why not do it? On 1/10/09, Steven Bethard wrote:

On Sat, Jan 10, 2009 at 7:35 AM, Gerald Britton wrote:

...

I would like to know if anyone has thought of adding a "while" clause as well, like this:

a = (i for i in range(100) while i <=50)

I think this could end up being confusing. Current generator expressions turn into an equivalent generator function by simply indenting the clauses and adding a yield, for example:

(i for i in range(100) if i % 2 == 0)

is equivalent to:

def gen(): for i in range(100): if i % 2 == 0: yield i

Now you're proposing syntax that would no longer work like this. Taking your example:

(i for i in range(100) while i <= 50)

I would expect this to mean:

def gen(): for i in range(100): while i <= 50: yield i

In short, -1. You're proposing to use an existing keyword in a new way that doesn't match how generator expressions are evaluated.

Steve -- I'm not *in*-sane. Indeed, I am so far *out* of sane that you appear a tiny blip on the distant coast of sanity. --- Bucky Katt, Get Fuzzy

-- Sent from my mobile device

Hmmm. I can't really see what you are saying. Your example doesn't quite get the intention. The "while" clause in the example would translate like this: def gen(): for i in range(100): if i <= 50: yield i else: break Also, I think that for a new python user, (n for n in range(100) while n*n < 50) is easier to understand and use than: (n for n in takewhile(lambda n:n*n < 50, range(100))) My proposed version is shorter (less chance for typos), has one less set of parentheses (always a good thing) and reads naturally. Also, it is directly analogous to: (n for n in range(100) if n*n < 50) except that the "while" version stops when n reaches 8. The "if" version doesn't stop until n reaches 99. On Sat, Jan 10, 2009 at 5:45 PM, Steven Bethard wrote:

[Fixing the top-posting. Note that only Guido is allowed to top-post around here. ;-)]

On 1/10/09, Steven Bethard wrote:

...

I think this could end up being confusing. Current generator expressions turn into an equivalent generator function by simply indenting the clauses and adding a yield, for example:

(i for i in range(100) if i % 2 == 0)

is equivalent to:

def gen(): for i in range(100): if i % 2 == 0: yield i

Now you're proposing syntax that would no longer work like this. Taking your example:

(i for i in range(100) while i <= 50)

I would expect this to mean:

def gen(): for i in range(100): while i <= 50: yield i

On Sat, Jan 10, 2009 at 2:34 PM, Gerald Britton wrote:

...

I guess I don't think it would be confusing. On the contrary, I believe that the expressions would read naturally and be a nice simplification. Of course it won't work just like "if" but that is just the point! I can (and do) accomplish the same thing with "takewhile", but if the same thing can be sone with a little addition to the generator expression, why not do it?

I'm probably just repeating myself here, but the reason not to do it is that the current generator expressions translate almost directly into the corresponding generator statements. Using "while" in the way you've suggested breaks this symmetry, and would make Python harder to learn.

Steve -- I'm not *in*-sane. Indeed, I am so far *out* of sane that you appear a tiny blip on the distant coast of sanity. --- Bucky Katt, Get Fuzzy

Gerald Britton schrieb:

Hmmm. I can't really see what you are saying. Your example doesn't quite get the intention. The "while" clause in the example would translate like this:

def gen(): for i in range(100): if i <= 50: yield i else: break

Also, I think that for a new python user,

(n for n in range(100) while n*n < 50)

is easier to understand and use than:

(n for n in takewhile(lambda n:n*n < 50, range(100)))

My proposed version is shorter (less chance for typos), has one less set of parentheses (always a good thing) and reads naturally.

The other is needlessly complicated, though. takewhile(lambda n: n*n < 50, range(100)) is just as fine, and only 3 characters longer than the proposed while-exp. Georg -- Thus spake the Lord: Thou shalt indent with four spaces. No more, no less. Four shall be the number of spaces thou shalt indent, and the number of thy indenting shall be four. Eight shalt thou not indent, nor either indent thou two, excepting that thou then proceed to four. Tabs are right out.