Removal of element from list while traversing causes the next element to be skipped

[William McBrine]

Look at this -- from Python 2.5.1:

>>> a = [1, 2, 3, 4, 5]
>>> for x in a:
...     if x == 3:
...         a.remove(x)
...     print x
... 
1
2
3
5
>>> a
[1, 2, 4, 5]
>>>

Sure, the resulting list is correct. But 4 is never printed during the 
loop!

What I was really trying to do was this:

apps = [name for name in os.listdir(ROOT) if 
        os.path.isdir(os.path.join(ROOT, name))]

apptitles = {}

for name in apps:
    try:
        app = __import__(name)
    except:
        apps.remove(name)
    else:
        apptitles[name] = getattr(app, 'TITLE', name.title())

which worked fine, until I actually had a directory with no module in it. 
Then that directory was correctly removed from the list, but the _next_ 
one was skipped, so its title was never assigned, which caused problems 
later in the program.

-- 
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0 -- pass it on