counting items

[Steven Bethard]

Mark McEahern wrote:
> It's me wrote:
> 
>> Okay, I give up.
>>
>> What's the best way to count number of items in a list [that may 
>> contain lists]?
>>  
>>
> a = [[1,2,4],4,5,[2,3]]
> 
> def iterall(seq):
>    for item in seq:
>        try:
>            for subitem in iterall(item):
>                yield subitem
>        except TypeError:
>            yield item
> 
> all = [x for x in iterall(a)]
> print len(all)
> 

Beware:

py> list(iterall(['a']))
Traceback (most recent call last):
   File "<interactive input>", line 1, in ?
   File "<interactive input>", line 4, in iterall
   ...
   File "<interactive input>", line 4, in iterall
RuntimeError: maximum recursion depth exceeded

You need to special-case strings.  I would do this explicitly, e.g.:

py> def iterall(seq):
...    for item in seq:
...        if isinstance(item, basestring):
...            yield item
...        else:
...            try:
...                for subitem in iterall(item):
...                    yield subitem
...            except TypeError:
...                yield item
...
py> list(iterall(['a']))
['a']

but you can also do this by checking for __iter__ as in one of the other 
posted solutions.  (This takes advantage of the fact that strings use 
the __getitem__ protocol for iteration instead of __iter__.)

Steve