A python function for two way dictionary transformations

transform.py

from collections import defaultdict

example_map= (
    ('n.given_name', 'first'),
    ('n.family_name', 'last'),
    ('tel.value', 'tel'),
    ('tel.type', 'teltype')
)

def transform(source, map_, reverse=False):
    infinite_defaults = lambda: defaultdict(infinite_defaults)
    dest = infinite_defaults()
    for attr in map_:
        if reverse:
            from_, to = attr
        else:
            to, from_ = attr
        from_ = from_.split('.')
        to = to.split('.')
        def _get(k, ob):
            if len(k) > 1:
                return _get(k[1:], ob[k[0]])
            else:
                return ob[k[0]]
        try:
            val = _get(from_, source)
        except KeyError:  # if we can't find a key in the source, then we skip it
            continue
        def _set(k, ob):
            if len(k) > 1:
                _set(k[1:], ob[k[0]])
            else:
                ob[k[0]] = val
        _set(to, dest)
    return dest