__eq__ and its effect on __hash__

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Python enforces the contract that objects which compare equal must have the same hash. If you define `__eq__` without `__hash__`, Python sets `__hash__ = None` on your class, making instances unhashable. Attempting to use them in a set or as dict keys then raises `TypeError: unhashable type`. To restore hashability, explicitly define `__hash__` - for example, by hashing a tuple of the fields that define equality.