Quite often I find myself wanting to write an infinite for-loop, or rather a loop with a counter, that terminates on some inner condition. Recently I even wanted to do this together with the generator comprehension syntax, I don't remember exactly what I wanted to do, but it was something like: zip( some_list_of_unknown_length, ('a' for x in infinit_generator) ) I admit that my example is silly, but it still serves as an example of what I wanted to do. Then I read that Ellipsis will become generally accepted in py3k [1] and thought why not let range accept ... as end-parameter to mean "until forever". More silly examples to demonstrate how it would work:
for i in range(...): ... print i ... if i == 4: break 0 1 2 3 4 for i in range(10,...,10): ... print i ... if i == 40: break 10 20 30 40
Any thoughts on this? /Tobias [1] http://mail.python.org/pipermail/python-3000/2006-April/000996.html
On 3/21/07, Tobias Ivarsson
Quite often I find myself wanting to write an infinite for-loop, or rather a loop with a counter, that terminates on some inner condition. Recently I even wanted to do this together with the generator comprehension syntax, I don't remember exactly what I wanted to do, but it was something like: zip( some_list_of_unknown_length, ('a' for x in infinit_generator) ) I admit that my example is silly, but it still serves as an example of what I wanted to do.
itertools.count() (http://docs.python.org/lib/itertools-functions.html) handles just this. Wrapping it in a genexp can take care of the second example:
for i in range(...): ... print i ... if i == 4: break
for i in itertools.count(): ....
for i in range(10,...,10): ... print i ... if i == 40: break
for i in (i * 10 for i in itertools.count(1)): .... Collin Winter
"Tobias Ivarsson"
Quite often I find myself wanting to write an infinite for-loop, or rather a loop with a counter, that terminates on some inner condition. Recently I even wanted to do this together with the generator comprehension syntax, I don't remember exactly what I wanted to do, but it was something like: zip( some_list_of_unknown_length, ('a' for x in infinit_generator) ) I admit that my example is silly, but it still serves as an example of what I wanted to do.
Then I read that Ellipsis will become generally accepted in py3k [1] and thought why not let range accept ... as end-parameter to mean "until forever".
More silly examples to demonstrate how it would work:
for i in range(...): ... print i ... if i == 4: break [snip] Any thoughts on this?
It isn't reasonable in Python 2.x, as range() returns an actual list, and an infinite list isn't reasonable. It would be *possible* with xrange() in Python 2.x, but then the question is "is such desireable?". The answer there is also no, as currently 2.x range() and xrange() are limited by your platform int size (generally 32 bits)... >>> xrange(2**31) Traceback (most recent call last): File "<stdin>", line 1, in <module> OverflowError: long int too large to convert to int >>> xrange(2**31-1) xrange(2147483647) >>> In Python 3, ints and longs will be unified, so the whole 'limited by platform int' shouldn't be applicable. Further, xrange is renamed to range. So it becomes more reasonable. On the other hand, a user (you) should be able to give a *huge* value to range, and it would work as you want ... to. Generally, you are pretty safe with 2**64 * increment, but if you are really anal, go with 2**128 * increment, that should keep you safe until the big freeze. Ultimately, -1. It's not usable in Python 2.x, and Python 3.x will support a variant trivially. - Josiah
Josiah Carlson wrote:
Generally, you are pretty safe with 2**64 * increment, but if you are really anal, go with 2**128 * increment, that should keep you safe until the big freeze.
Although this will work for all practical purposes, code which does things like this is still a bit smelly. I prefer it when there are ways of explicitly representing infinitely-large or unbounded values. -- Greg
participants (4)
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Collin Winter -
Greg Ewing -
Josiah Carlson -
Tobias Ivarsson