[Python-ideas] "While" suggestion

[Stavros Korokithakis]

That is quite a bit better than duplicating the assignment, but this 
version duplicates the check (or rather defers it from the "while 
<expression>" part to the "if" part). I am not sure that it is as 
elegant as the proposed syntax...

Stavros

Chris Rebert wrote:
> The alternate idiom I've seen for "do-while"-like loops such as this
> in Python is (using the original example):
> 
> while True:
>     data = my_file.read(1024)
>     if not data: break
>     do_something(data)
> 
> which seems perfectly acceptable and in keeping with TOOWTDI by
> minimizing the number of loop constructs in the language.
> It obviously avoids the distasteful duplication of "data = my_file.read(1024)".
> 
> - Chris
> 
> On Thu, Jul 3, 2008 at 7:58 AM, Thomas Lee <tom at vector-seven.com> wrote:
>> Stavros Korokithakis wrote:
>>> I don't think they do, if I'm not mistaken the only way is to call read()
>>> and see if it returns the empty string. I agree that this would be better,
>>> but the use case I mentioned is not the only one this would be useful in...
>>> Unfortunately I can't think of any right now,
>> :)
>>> but there have been a few times when I had to initialise things outside
>>> the loop and it always strikes me as ugly.
>>>
>> Well that depends, on the situation really. The only use case I can think of
>> is exactly the one you mentioned. And since you can't think of any other
>> scenarios where such a thing might be handy, I've got no better suggestion
>> to offer.
>>
>> If you can conjure up another scenario, post it back here and we'll see if
>> we can generalize the pattern a little.
>>
>> In the meantime, I'd love a way to check if a file is at its end without
>> having to read data out of it ...
>>
>> Cheers,
>> T
>>
>> P.S. you might be interested in using something like the following to hide
>> the ugliness in a function:
>>
>> def reader(stream, size):
>>  data = stream.read(size)
>>  while data:
>>   yield data
>>   data = stream.read(size)
>>
>> Then you could use it as follows:
>>
>> for block in reader(my_file, 1024):
>>  do_something(block)
>>> Thomas Lee wrote:
>>>> It's not currently possible to determine if a file/stream is at its end,
>>>> is it?
>>>>
>>>> If that were the case you could easily do the read before your
>>>> do_something call. Something like:
>>>>
>>>> while not my_file.eof:
>>>>  data = my_file.read(1024)
>>>>  do_something(data)
>>>>
>>>> Can anyone explain why file objects don't support some sort of eof check?
>>>> Something gives me the impression that it was an intentional decision.
>>>>
>>>> IMO something like this would be better than more syntax.
>>>>
>>>> Cheers,
>>>> T
>>>>
>>>> Stavros Korokithakis wrote:
>>>>> Hello all,
>>>>> I have noticed that sometimes "while" loops produce "unpythonic"
>>>>> patterns, such as the following:
>>>>>
>>>>> data = my_file.read(1024)
>>>>> while data:
>>>>>    do_something(data)
>>>>>    data = my_file.read(1024)
>>>>>
>>>>> The assignment is repeated, which is less than optimal. Since we don't
>>>>> have a while statement that does the check in the end, would it not be
>>>>> better if the syntax of while could be amended to  include something like
>>>>> this (in the spirit of the new "with" keyword)?:
>>>>>
>>>>> while my_file.read(1024) as data:
>>>>>   do_something(data)
>>>>>
>>>>> This would terminate the loop when myfile.read() evaluated to False, and
>>>>> it is more pythonic than repeating onesself.
>>>>>
>>>>> I contacted GvR about this, and he replied that this syntax would have
>>>>> to be part of the expression than part of the while, which I agree would be
>>>>> less than ideal. However, I don't see why it would have to be part of the
>>>>> expression, since the "while" could easily assign the value of the
>>>>> expression to the variable and break if it evaluates to False.
>>>>>
>>>>> I would appreciate any thoughts on this,
>>>>> Stavros Korokithakis
>>>>> ------------------------------------------------------------------------
>>>>>
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>>>>> Python-ideas at python.org
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>>>>>
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