'Get the value of a cached object.
Parameters
key : any
The key to lookup.
dt : datetime
The time of the lookup.
Returns
result : any
The value for ``key``.
Raises
KeyError
Raised if the key is not in the cache or the value for the key
has expired.'

def get(self, key, dt):

    try:
        return self._cache[key].unwrap(dt)
    except Expired:
        del self._cache[key]
        raise KeyError(key)
