[Python-ideas] BUG in standard while statement

[Stephan Sahm]

It is so true!
thanks for pointing that out. It makes sense to do it that way
I probably never used while in different places than ``while 1: pass``
until now.

I admit, I am looking for something alternative to a for structure like
 ``for _ in range(10)`` -- I don't like the ``_`` ;-)
How can I use the iterator protocoll to make a nice repeat syntax?


On 9 September 2015 at 19:15, Joseph Jevnik <joejev at gmail.com> wrote:

> This appears as intended. The body of the while condition is executed each
> time the condition is checked. In the first case, you are creating a single
> instance of repeat, and then calling bool on the expression with each
> iteration of the loop. With the second case, you are constructing a _new_
> repeat instance each time. Think about the difference between:
>
> while should_stop():
>     ...
>
> and:
> a = should_stop()
> while a:
>     ...
>
> One would expect should_stop to be called each time in the first case;
> but, in the second case it is only called once.
>
> With all that said, I think you want to use the __iter__ and __next__
> protocols to implement this in a more supported way.
>
> On Wed, Sep 9, 2015 at 1:10 PM, Stephan Sahm <Stephan.Sahm at gmx.de> wrote:
>
>> Dear all
>>
>> I found a BUG in the standard while statement, which appears both in
>> python 2.7 and python 3.4 on my system.
>>
>> It usually won't appear because I only stumbled upon it after trying to
>> implement a nice repeat structure. Look:
>> ​```​
>> class repeat(object):
>>     def __init__(self, n):
>>         self.n = n
>>
>>     def __bool__(self):
>>         self.n -= 1
>>         return self.n >= 0
>>
>>     __nonzero__=__bool__
>>
>> a = repeat(2)
>> ```
>> the meaning of the above is that bool(a) returns True 2-times, and after
>> that always False.
>>
>> Now executing
>> ```
>> while a:
>>     print('foo')
>> ```
>> will in fact print 'foo' two times. HOWEVER ;-) ....
>> ```
>> while repeat(2):
>>     print('foo')
>> ```
>> will go on and go on, printing 'foo' until I kill it.
>>
>> Please comment, explain or recommend this further if you also think that
>> both while statements should behave identically.
>>
>> hoping for responses,
>> best,
>> Stephan
>>
>>
>>
>> _______________________________________________
>> Python-ideas mailing list
>> Python-ideas at python.org
>> https://mail.python.org/mailman/listinfo/python-ideas
>> Code of Conduct: http://python.org/psf/codeofconduct/
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20150909/7452c117/attachment.html>