[Python-Dev] Dataclasses and correct hashability

[Guido van Rossum]

On Tue, Feb 6, 2018 at 11:40 AM, Ethan Furman <ethan at stoneleaf.us> wrote:

> It sounds like `unsafe_hash=True` indicates a truly unsafe hash (that is,
> mutable data is involved in the hash calculation), but there still seems to
> be one possibility for an "unsafe_hash" to actually be safe -- that is, if
> only immutable fields are used in __eq__, then dataclass could safely
> generate a hash for us.
>
> Do we have a way to know if the equality fields are hashable?  I suppose
> we could check each one for a for a non-None __hash__.  Then we could
> modify that first condition from
>
> - frozen=True
>
> to
>
> - frozen=True or all(getattr(eq_fld, '__hash__', None) is not None for
> eq_field in equality_fields)
>

There seems to be a misunderstanding underlying these questions. Even if
all fields have an immutable type (e.g. all ints, supporting __eq__ and
__hash__), if the containing class isn't frozen, they can be assigned to.
E.g.

@dataclass()
class Point:
    x: int
    y: int

p = Point(1, 1)
p.x = 2  # This is legal

The only way to make that assignment to p.x illegal is to make the *class*
frozen (using @dataclass(frozen=True)) -- nothing we can do about the
*field* will change this.

Of course if you use @dataclass(frozen=True, unsafe_hash=True) you may
still get a safe hash. :-)

-- 
--Guido van Rossum (python.org/~guido)
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