Can you replace a dictionary value with one key lookup?

Question

I would like to know whether there is a more efficient way of replacing a particular value in a dictionary. By "more efficient" I mean to avoid looking up the same key twice. I don't expect it would make a big difference, but I find myself doing the following alot:

foo = {'a': 0, 'b': 1}
foo['b'] = bar(foo['b'])

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Update

I think the assignment above is "looking up the same key twice" because the following prints "Hashing b" three times.

class LoudKey:
    def __init__(self, key):
        self.key = key
    def __hash__(self):
        print(f'Hashing {self.key}')
        return self.key.__hash__()

b = LoudKey('b')
foo = {'a': 0, b: 1}
# first "Hashing b"
foo[b] = float(foo[b])
# next two "Hashing b"s

If dict.__getitem__ and dict.__setitem__ are really not duplicating effort somehow, an elaboration on that would also be accepted as an answer.

Answer 1

You can do it by making the dictionary items a mutable type, then mutating it. The simplest is a list with a single element.

>>> b = LoudKey('b')
>>> foo = {'a': [0], b: [1]}
Hashing b
>>> ref = foo[b]
Hashing b
>>> ref[0] = float(ref[0])
>>> foo
{'a': [0], <__main__.LoudKey object at 0x0000000014C66F40>: [1.0]}

You're right that it wouldn't make much difference in practice, though.