Finding items not in 2 lists/dictionaries

[Robin Siebler]

I have no clue what the '&c' means or does.  Also, the lists are
actually, quite long (I am comparing 2 directory trees).  Would the
solution on the bottom of page 54 be a faster/better way to do this?

Alex Martelli <aleax at aleax.it> wrote in message news:<6Qosa.53623$K35.1499331 at news2.tin.it>...
> <posted & mailed>
> 
> Robin Siebler wrote:
> 
> > On page 15 of the Python Cookbook, it shows how to find all of the
> > common items in 2 dictionaries.  How would I find all of the items
> > that are *not* in both dictionaries (or lists)?
> 
> The CB p15 solution actually deals with keys, as the problem statement
> shows, although the discussion does debatably talk of items.  Anyway,
> 
> [k for k in dict1 if k not in dict2] + [k for k in dict2 if k not in dict1]
> 
> is the "one obvious way" to get a list of keys that are in either one dict 
> and not in the other.  Beware: this would be SLOW for two long lists -- 
> making dicts out of the lists (e.g. with Python 2.3 dict.fromkeys, or in 
> 2.2 with auxdict1=dict(zip(list1,list1)) &c, is likely to speed things up
> dramatically unless the lists are quite short.
> 
> 
> Alex