A dicts diff and patch utilities

diffs.py

def diff(old, new):
    """
    diff(a, b) = b - a
    a + diff(a, b) == b
    """
    return strip_dict(map_dicts(diff_atom, old, new))

def diff_atom(old, new):
    return None if old == new else (old, new)

def patch(old, diff): # -> new
    return map_dicts(atom_patch, old, diff)

def atom_patch(value, (old, new)):
    assert value == old
    return new

def patchback(new, diff): # -> old
    pass

### tools

def strip_dict(d):
    return {k: v for k, v in d.iteritems() if v is not None}

def map_dicts(func, d1, d2):
    return {key: func(v1, v2) for key, v1, v2 in iterate_dicts(d1, d2)}

def iterate_dicts(d1, d2):
    keys = set(d1) | set(d2)
    for key in keys:
        yield key, d1.get(key), d2.get(key)