Hashable Objects

An object is hashable if it has a hash code which never changes during its lifetime (it needs a __hash__() method), and can be compared to other objects (it needs an __eq__() method). Hashable objects which compare equal must have the same hash code.

Hashable Types

1. Numeric types;
2. Flat immutable¹ types str and bytes;
3. Container types¹ if the object and all elements are immutable and hashable:
   • FrozenSet are hashable because every element it contains must be hashable by definition;
   • A Tuple is hashable only if all its items are hashable;

Characteristics

1. The hash code might vary based on the Python version, machine setup, and the way the hash was created;
2. The hase code is guaranteed to be the same only within one Python process;
3. Custom user-defined types are hashable by default. This happens because their hash code is derived from their id(), and the __eq__() method, which is inherited from the object class, simply checks whether the object IDs are the same;
4. If an object implements a custom __eq__() that takes into account its internal state, it will be hashable only if its __hash__() always returns the same hash code:
   • In practice, this requires that __eq__() and __hash__() only take into account instance attributes that never change during the life of the object

Code

hashable_obj.py

>>> tt = (1, 2, (30, 40))
>>> print(hash(tt))
>>> tl = (1, 2, [30, 40])
>>> try:
...     hash(tl)
... except BaseException as ex:
...     print(f"Hash error when use unhashable obj: {ex}")
>>> tf = (1, 2, frozenset([30, 40]))
>>> print(hash(tf))
-3907003130834322577
Hash error when use unhashable obj: unhashable type: 'list'
5149391500123939311

References

• Fluent Python, 2nd Edition

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1. Flat vs Container debate ↩↩