merging two python dicts and keeping the max key, val in the new updated dict

Question

I need a method where I can merge two dicts keeping the max value when one of the keys, value are in both dicts.

dict_a maps \"A\", \"B\", \"C\" to 3, 2, 6

dict_

Answer 1

OK so this works by making a union set of all possible keys dict_a.keys() | dict_b.keys() and then using dict.get which by default returns None if the key is not present (rather than throwing an error). We then take the max (of the one which isn't None).

def none_max(a, b):
    if a is None:
        return b
    if b is None:
        return a
    return max(a, b)

def max_dict(dict_a, dict_b):
   all_keys = dict_a.keys() | dict_b.keys()
   return  {k: none_max(dict_a.get(k), dict_b.get(k)) for k in all_keys}

Note that this will work with any comparable values -- many of the other answers fail for negatives or zeros.

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Example: Inputs:

dict_a = {'a': 3, 'b': 2, 'c': 6}

dict_b = {'b': 7, 'c': 4, 'd': 1}

Outputs:

max_dict(dict_a, dict_b)  # == {'b': 7, 'c': 6, 'd': 1, 'a': 3}