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Wed Mar 16 14:37:24 CET 2005 · https://mail.python.org/pipermail/python-dev/2005-March/052220.html

Bob Ippolito wrote:
> On Mar 16, 2005, at 6:19, Raymond Hettinger wrote:
> 
>> Some folks on comp.lang.python have been pushing for itertools to
>> include a flatten() operation.  Unless you guys have some thoughts on
>> the subject, I'm inclined to accept the request.
>>
>> Rather than calling it flatten(), it would be called "walk" and provide
>> a generalized capability to descend through nested iterables (similar to
>> what os.walk does for directories).  The one wrinkle is having a
>> stoplist argument to specify types that should be considered atomic
>> eventhough they might be iterable (strings for example).
> 
> 
> You could alternatively give them a way to supply their own "iter" 
> function, like the code I demonstrate below:

I think the extra flexibility ends up making the function harder to comprehend 
and use. Here's a version with a simple stoplist:

from itertools import chain

def walk(iterable, atomic_types=(basestring,)):
   itr = iter(iterable)
   while True:
     for item in itr:
       if isinstance(item, atomic_types):
         yield item
         continue
       try:
         subitr = iter(item)
       except TypeError:
         yield item
       else:
         itr = chain(walk(subitr), itr)
         break
     else:
       break

This makes it easy to reclassify certain things like dictionaries or tuples as 
atomic elements.

 > # maybe there should be a bfswalk too?

By putting the 'walk(subitr)' after the current itr when chaining?

If Raymond does decide to go for the flexible approach rather than the simple 
one, then I'd vote for a full-featured approach like:

def walk(iterable, depth_first=True, atomic_types=(basestring,), iter_factory=iter):
   itr = iter(iterable)
   while True:
     for item in itr:
       if isinstance(item, atomic_types):
         yield item
         continue
       try:
         subitr = iter_factory(item)
       except TypeError:
         yield item
       else:
         if depth_first:
           itr = chain(walk(subitr), itr)
         else:
           itr = chain(itr, walk(subitr))
         break
     else:
       break

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at email.com   |   Brisbane, Australia
---------------------------------------------------------------
             http://boredomandlaziness.skystorm.net

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[Python-Dev] itertools.walk()