How to test if one dict is subset of another?

[Paul Rubin]

"Jay Tee" <jeff.templon at gmail.com> writes:
> Python 2.2.3 (#1, Oct 26 2003, 11:49:53)
> ImportError: No module named sets

Hmm, well I think the sets module in 2.3 is written in Python, so you
could drop it into your application for use in 2.2.  Better would be
to use the C version from 2.4 if you can.  Or you could fake it with
dicts.  Sets are really just dicts under the skin.  Instead of
set([1,2,3]) you'd use {1:True, 2:True, 3:True}.  To intersect
two of them (untested):

def intersection(d1,d2):
   if len(d2) < len(d1):
     # swap them so d1 is the smaller one
     d1,d2 = d2,d1
   r = {}
   for k in d1.iterkeys():
     if k in d2:
       r[k] = True
   return r

The d1,d2 swap is just an optimization, since access is constant-time
you want to scan the smaller dictionary to minimize the number of
lookups.