> The code in msg311969 doesn't reuse hash values.

That doesn't make sense.  The dict.update() method reuses the hashes of the input mappings when possible.

    >>> from collections import ChainMap
    >>> class Int(int):
            def __hash__(self):
                    print(f'Hashing {self}', file=sys.stderr)
                    return int.__hash__(self)

            
    >>> import sys
    >>> d = { Int(1): 'f1', Int(2): 'f2' }
    Hashing 1
    Hashing 2
    >>> e = { Int(1): 's1', Int(3): 's3' }
    Hashing 1
    Hashing 3
    >>> c = ChainMap(d, e)
    >>> list(c)                     # Note, no calls to hash() were made
    [1, 3, 2]