Hashing

Hash Method Generation

Warning

The overarching theme is to never set the @attr.s(hash=X) parameter yourself. Leave it at None which means that attrs will do the right thing for you, depending on the other parameters:

  • If you want to make objects hashable by value: use @attr.s(frozen=True).
  • If you want hashing and comparison by object identity: use @attr.s(cmp=False)

Setting hash yourself can have unexpected consequences so we recommend to tinker with it only if you know exactly what you’re doing.

Under certain circumstances, it’s necessary for objects to be hashable. For example if you want to put them into a set or if you want to use them as keys in a dict.

The hash of an object is an integer that represents the contents of an object. It can be obtained by calling hash() on an object and is implemented by writing a __hash__ method for your class.

attrs will happily write a __hash__ method for you [1], however it will not do so by default. Because according to the definition from the official Python docs, the returned hash has to fulfill certain constraints:

  1. Two objects that are equal, must have the same hash. This means that if x == y, it must follow that hash(x) == hash(y).

    By default, Python classes are compared and hashed by their id(). That means that every instance of a class has a different hash, no matter what attributes it carries.

    It follows that the moment you (or attrs) change the way equality is handled by implementing __eq__ which is based on attribute values, this constraint is broken. For that reason Python 3 will make a class that has customized equality unhashable. Python 2 on the other hand will happily let you shoot your foot off. Unfortunately attrs currently mimics Python 2’s behavior for backward compatibility reasons if you set hash=False.

    The correct way to achieve hashing by id is to set @attr.s(cmp=False). Setting @attr.s(hash=False) (that implies cmp=True) is almost certainly a bug.

  2. If two object are not equal, their hash should be different.

    While this isn’t a requirement from a standpoint of correctness, sets and dicts become less effective if there are a lot of identical hashes. The worst case is when all objects have the same hash which turns a set into a list.

  3. The hash of an object must not change.

    If you create a class with @attr.s(frozen=True) this is fullfilled by definition, therefore attrs will write a __hash__ function for you automatically. You can also force it to write one with hash=True but then it’s your responsibility to make sure that the object is not mutated.

    This point is the reason why mutable structures like lists, dictionaries, or sets aren’t hashable while immutable ones like tuples or frozensets are: point 1 and 2 require that the hash changes with the contents but point 3 forbids it.

For a more thorough explanation of this topic, please refer to this blog post: Python Hashes and Equality.

Hashing and Mutability

Changing any field involved in hash code computation after the first call to __hash__ (typically this would be after its insertion into a hash-based collection) can result in silent bugs. Therefore, it is strongly recommended that hashable classes be frozen.

Hash Code Caching

Some objects have hash codes which are expensive to compute. If such objects are to be stored in hash-based collections, it can be useful to compute the hash codes only once and then store the result on the object to make future hash code requests fast. To enable caching of hash codes, pass cache_hash=True to @attrs. This may only be done if attrs is already generating a hash function for the object.

[1]The hash is computed by hashing a tuple that consists of an unique id for the class plus all attribute values.