[Python-ideas] "While" suggestion

Andrew Akira Toulouse andrew at atoulou.se
Fri Jul 11 11:10:48 CEST 2008

Clearly, the list comprehension is intended to map and filter in a
functional matter. My [-1] operation was a trivial reduce operation.
Certainly another example could be concocted to demonstrate it used in such
a matter that would not offend your sensibilities, but as far as I'm
concerned it is simple, beautiful, readable, unambiguous, etc.

Your counterpoint takes exception to the example rather than the suggestion.
Say someone is going through a flat file with a large list of numbers, and
they want to filter out all odd numbers, and stop once they encounter a 0

[int(line) for line in file if evenP(int(line)) until int(line)==0]


[int(line) for line in file if evenP(int(line)) until int(line)==0 with
open(filename) as file]

Bam, one line to open the file, iterate through them, filter out unneeded
entries, define the base case, and return value.

Do you believe that this also abuses list comprehensions? If so, please
explain your reasoning.

There's nothing keeping a programmer from writing anything this syntax could
do as a normal for loop, but neither is there anything stopping said
programmer from using a for loop with ifs and breaks in place of a list

One can also make the reasoning that for infinitely iterable sequences, in
order for the shorter syntax to remain useful it should be able to define a
stopping point at which it ceases to map over said sequence. This is my
justification for suggesting 'until'; say someone created a function that
yielded the fibonacci numbers: it would be easy to iterate over this with a
full for statement and forget or misplace the base case which in turn would
lead to an infinite loop. 'until' offers a relatively simple solution that
at worst adds a few words to a program; on average, could reduce a little
bit of processing time; and at best, prevents an infinite for loop -- all in
a simple, readable, and compact syntax that, as far as I know (I admit I am
very new to python-ideas), would add little complexity to the interpreter.


On Wed, Jul 9, 2008 at 4:59 PM, Greg Ewing <greg.ewing at canterbury.ac.nz>

> Andrew Toulouse wrote:
>  tokenchecks = [token for regex,token in match_tok until regex.match(s)]
>> # do whatever from this point forward.
>>    return tokenchecks[-1]
> This is an abuse of list comprehensions, since
> you're throwing away the produced list and only
> keeping the last element.
> --
> Greg
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> Python-ideas at python.org
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