How do you compare 2 dictionaries to know if they have the same item or key?

I think the simplest way would be:

dic1 = {'a': 2, 'e': 5, 'u': 1}
dic2 = {'a': 3, 'e': 2, 'i': 2}

newdict = { k: dic1.get(k, 0) + dic2.get(k, 0) for k in set(dic1) | set(dic2) }
print(newdict)

{'a': 5, 'i': 2, 'u': 1, 'e': 7}

That is:

• We combine all the keys by set(dic1) | set(dic2)
• We go through them and add between both dictionaries. When using get() with a default value, there is no problem if a key does not exist in one of the two dictionaries

If you want to obtain all the unique keys present in at least one of the dictionaries you can use the union of sets:

>>> set(dic1) | set(dic2)
{'a', 'e', 'u', 'i'}

What you really want is to join both dictionaries and add the values, you can apply the union of sets together with dict.get in a dictionary by compression:

dic1 = {'a': 2, 'e': 5, 'u': 1}
dic2 = {'a': 3, 'e': 2, 'i': 2}


res = {key: dic1.get(key, 0) + dic2.get(key, 0) for key in set(dic1) | set(dic2)}

>>> res
{'u': 1, 'a': 5, 'e': 7, 'i': 2}

The construction of the dictionary by compression is efficient, the problem is that iteration is required twice for each dictionary, in addition to the construction of a set for each and a final result of the union. This makes for extended dictionaries more efficient in terms of memory and at run time from a certain point iterate over each dictionary sequentially:

from collections import defaultdict

res = defaultdict(int)
for dict_ in (dic1, dic2):
    for key, value in dict_.items():
        res[key] += value

>>> res
defaultdict(<class 'int'>, {'a': 5, 'e': 7, 'u': 1, 'i': 2})
>>> dict(res)
{'a': 5, 'e': 7, 'u': 1, 'i': 2}

The partners' solutions are really good. I can think of another one that is less elegant and concise because it does not use compression, but that improves efficiency in terms of execution times:

dic1 = {'a': 2, 'e': 5, 'u': 1}
dic2 = {'a': 3, 'e': 2, 'i': 2}

dic3 = dic1.copy()

for k, v in dic2.items():
    dic3[k] = v if k not in dic3.keys() else v + dic3[k]

What works:

>>> dic3
{'a': 5, 'u': 1, 'e': 7, 'i': 2}