On Mar 8, 2:43 pm, malkarouri <[EMAIL PROTECTED]> wrote:
> Hi everyone,
>
> I have an algorithm in which I need to use a loop over a queue on
> which I push values within the loop, sort of:
>
> while not(q.empty()):
> x = q.get()
> #process x to get zero or more y's
> #for each y:
> q.put(y)
>
> The easiest thing I can do is use a list as a queue and a normal for
> loop:
>
> q = [a, b, c]
>
> for x in q:
> #process x to get zero or more y's
> q.append(y)
>
> It makes me feel kind of uncomfortable, though it seems to work. The
> question is: is it guaranteed to work, or does Python expect that you
> wouldn't change the list in the loop?
>
> Regards,
>
> Muhammad Alkarouri
You could make it safe this way ('emulating' what a listiterator
object does in CPython):
>>> from itertools import count
>>> def index_iter(l):
... try:
... for i in count(): yield l[i]
... except IndexError:
... return
...
>>> l = list('SPAM')
>>> for c in index_iter(l):
... if c == 'A': l += list(' EGGS')
... if c == 'S': l += list(' NI')
...
>>> ''.join(l)
'SPAM NI EGGS NI'
>>>
Of course in practice this is no different from doing 'for c in l:...'
but it is safe against implementation changes.
--
Arnaud
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