I think these are an extremely common needs that are worth having standard 
methods for. If adding instance methods seems like a bad idea, then maybe add 
functions to the standard library that perform the same operations.

m = {'a': 123, 'b': 456, 'c': 789}
m.except(('a', 'c'))  # {'b': 456}
m.only(('b', 'c'))  # {'b': 456, 'c': 789}
m.values_at(('a', 'b'))  # [123, 456]

…or…

from mappings import except, only, values_at

m = {'a': 123, 'b': 456, 'c': 789}
except(m, ('a', 'c'))  # {'b': 456}
only(m, ('b', 'c'))  # {'b': 456, 'c': 789}
values_at(m, ('a', 'b'))  # [123, 456]
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