On 2/1/2018 7:34 PM, Elvis Pranskevichus wrote:
There appears to be a critical omission from the current dataclass
implementation: it does not make hash=True fields immutable.

Per Python spec:

"the implementation of hashable collections requires that a key’s hash
value is immutable (if the object’s hash value changes, it will be in
the wrong hash bucket)"

Yet:

     import dataclasses

     @dataclasses.dataclass(hash=True)
     class A:
         foo: int = dataclasses.field(hash=True, compare=True)

     a = A(foo=1)

     s = set()
     s.add(a)   # s == {a}
     a.foo = 2

     print(a in s)
     print({a} == s}
     print(s == s)

     # prints False False True


This looks to me like a clearly wrong behavior.


                                     Elvis

Data classes do not protect you from doing the wrong thing. This is the same as writing:

class A:
    def __init__(self, foo):
        self.foo = foo
    def __hash__(self):
        return hash((self.foo,))

You're allowed to override the parameters to dataclasses.dataclass for cases where you know what you're doing. Consenting adults, and all.

Eric.
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