dictionary with tuple keys

[John Machin]

Ben Finney <ben+python <at> benfinney.id.au> writes:
 
> In this case, I'll use ‘itertools.groupby’ to make a new sequence of
> keys and values, and then extract the keys and values actually wanted.

Ah, yes, Zawinski revisited ... itertools.groupby is the new regex :-)
 
> Certainly it might be clearer if written as one or more loops, instead
> of iterators. But I find the above relatively clear, and using the
> built-in iterator objects will likely make for a less buggy
> implementation.

Relative clarity like relative beauty is in the eye of the beholder,
and few parents have ugly children :-)

The problem with itertools.groupby is that unlike SQL's "GROUP BY"
it needs sorted input. The OP's requirement (however interpreted)
can be met without sorting.

Your interpretation can be implemented simply:

from collections import defaultdict
result = defaultdict(list)
for key, value in foo.iteritems():
    result[key[:2]].append(value)