Because -1 is not equal to -2. They will fall into the same bucket of the hash map, but the hash collisions are expected and must be accounted for.

Since hashes aren't guaranteed to be unique, a dictionary should use equality to actually confirm that the object with the given hash matches the key present in the dictionary.

> The __hash__() method should return an integer. The only required property is that objects which compare equal have the same hash value; it is advised to mix together the hash values of the components of the object that also play a part in comparison of objects ...

-- https://docs.python.org/3/reference/datamodel.html#object.__...

> The general contract of hashCode is:

> * If two objects are equal according to the equals method, then calling the hashCode method on each of the two objects must produce the same integer result.

> * It is not required that if two objects are unequal according to the equals method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

-- https://docs.oracle.com/en/java/javase/23/docs/api/java.base...

If hashmaps only looked at hash(k1) == hash(k2) and not also k1 == k2 they would be quite useless as maps.