[Python-Dev] Dataclasses and correct hashability

[Kirill Balunov]

>
> On Sun, Feb 4, 2018, 9:50 PM Guido van Rossum <guido at python.org <https://mail.python.org/mailman/listinfo/python-dev>> wrote:
>
> Looks like this is turning into a major flamewar regardless of what I say.
> :-(
> I really don't want to lose the ability to add a hash function to a
> mutable dataclass by flipping a flag in the decorator. I'll explain below.
> But I am fine if this flag has a name that clearly signals it's an unsafe
> thing to do.
>
> I propose to replace the existing (as of 3.7.0b1) hash= keyword for the
> @dataclass decorator with a simpler flag named unsafe_hash=. This would be
> a simple bool (not a tri-state flag like the current hash=None|False|True).
> The default would be False, and the behavior then would be to add a hash
> function automatically only if it's safe (using the same rules as for
> hash=None currently). With unsafe_hash=True, a hash function would always
> be generated that takes all fields into account except those declared using
> field(hash=False). If there's already a `def __hash__` in the function I
> don't care what it does, maybe it should raise rather than quietly doing
> nothing or quietly overwriting it.
>
> Here's my use case.
>
>
May be it is better to provide a special purpose function
`make_unsafe_hash` in
dataclass module which will patch a dataclass, instead of to clutter
@dataclass
API with arguments which are rather special case.

This `unsafe_hash` argument will constantly raise questions among ordinary
users
like me, and will be possibly considered as a non-obvious design - there is
a
public API but it is somehow unsafe. On the other hand, with a function,
when
the user asks how to make a `frozen=False` dataclass hashable, you can
suggest
to use this `make_unsafe_hash` function with all its cautions in its docs
or to try to
implement __hash__ by yourself.

Also taking into account the Python approach for backward compatibility it
is
better to stick with function and if it will be usefull to add a
`unsafe_hash`
argument in Python 3.8. It is easier to add later than to deprecate in the
future.

With kind regards,
-gdg
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