How to merge 2 dictionaries in Python?

There are multiple ways to achieve that but my favorite is the following:

dict_1 = {'a': 10, 'b': 20, 'c': 30}
dict_2 = {'c': 40, 'd': 50, 'e': 60}

merged_dict = {dict_key: dict_1.get(dict_key, 0) + dict_2.get(dict_key, 0) for dict_key in set(dict_1) | set(dict_2)}
assert merged_dict == {'a': 10,  'b': 20, 'c': 70,'d': 50, 'e': 60}

But with this structure you can do more things like merge only the entries defined in one of dictionaries:

merged_dict_1 = {dict_key: dict_1.get(dict_key, 0) + dict_2.get(dict_key, 0) for dict_key in set(dict_1)}
assert merged_dict_1 == {'a': 10, 'b': 20, 'c': 70}

Or keep only the common entries:

common_dict = {dict_key: dict_1.get(dict_key, 0) + dict_2.get(dict_key, 0) for dict_key in set(dict_1) & set(dict_2)}
assert common_dict == {'c': 70}