A Bidirectional Dictionary

Implement BiDict, a mapping where values are unique and you can look up either way:

• b[key] = value, b[key], del b[key], len(b), key in b, iteration over keys
• b.inverse[value] returns the key; b.inverse is itself a BiDict view that stays in sync (setting b.inverse[v] = k also updates b)
• assigning a value that already belongs to another key raises ValueError; re-assigning a key to a new value removes the old value from the inverse

Subclass collections.abc.MutableMapping so you get get, items, update and friends for free.