def diff(old, new):
"""Returns the set of differences between two C{dict}s.

@return: A 3-tuple of dicts with the changes that would need to be
made to convert C{old} into C{new}: C{(creates, updates, deletes)}
"""
new_keys = set(new.iterkeys())
old_keys = set(old.iterkeys())

creates = {}
for key in new_keys - old_keys:
creates[key] = new[key]

updates = {}
for key in old_keys & new_keys:
if old[key] != new[key]:
updates[key] = new[key]

deletes = {}
for key in old_keys - new_keys:
deletes[key] = old[key]

return creates, updates, deletes