# two dictionaries # blow this up to 1000 to see the difference some_dict = { 'zope':'zzz', 'python':'rocks' } another_dict = { 'python':'rocks', 'perl':'$' } # bad way # two lots of "in" intersect = [] for item in some_dict.keys(): if item in another_dict.keys(): intersect.append(item) print "Intersects:", intersect # good way # use simple lookup with has_keys() intersect = [] for item in some_dict.keys(): if another_dict.has_key(item): intersect.append(item) print "Intersects:", intersect