May I loop over a changing list?

[Olaf Delgado]

Hi folks!

I am whondering whether the following is considered good (and,
more important: save) programming style in python:

>>> def countdown(n):
...   a = [ n ]
...   for x in a:
...     if x > 0:
...       a.append(x-1)
...   return a
... 
>>> countdown(10)
[10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

If my understanding of what 'for x in y' is meant to do is correct, the
for loop should be equivalent to something like

i = 0
while i < len(a):
  x = a[i]
  if x > 0:
    a.append(x-1)
  i = i+1

I think appending to a list while looping over it is a neat way to emulate
a simple queue. The actual example is not quite clever, of course. I
actually use similar pieces of code for doing breadth-first traversals of
graphs.

Cheers,
Olaf