use dict.get() to increase value

a_dict = {"a":1}

a_dict["b"] = a_dict.get("b", 0) + 1

print(a_dict)
> {'a': 1, 'b': 1}

a_dict["a"] = a_dict.get("a", 0) + 1

print(a_dict)
> {'a': 2, 'b': 1}