[Python-ideas] BUG in standard while statement
Stephan.Sahm at gmx.de
Wed Sep 9 19:20:53 CEST 2015
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``
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():
> 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
>> 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:
>> will in fact print 'foo' two times. HOWEVER ;-) ....
>> while repeat(2):
>> 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,
>> Python-ideas mailing list
>> Python-ideas at python.org
>> Code of Conduct: http://python.org/psf/codeofconduct/
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Python-ideas