On 02/06/2018 11:18 AM, Guido van Rossum wrote:
We may be in violent agreement.
I propose *not* to add a way to *disable* hashing when the rest of the flags to
@dataclass() would indicate that it's
safe to add a __hash__ method.
Okay.
I propose that with @dataclass(unsafe_hash=False) (the default), a __hash__
method is added when the following
conditions are true:
- frozen=True (not the default)
- compare=True (the default)
- no __hash__ method is defined in the class
If we instead use @dataclass(unsafe_hash=True), a __hash__ will be added
regardless of the other flags, but if a
__hash__ method is present, an exception is raised.
Other values (e.g. unsafe_hash=None) are illegal for this flag.
Ah! Excellent, that greatly allays my worries.
Note that the the hash= flag to the field() function is unchanged from what's
currently in PEP 557 or in the
implementation in 3.7.0b1. In particular, the generated __hash__ method will
disregard fields declared using
field(hash=False). It will also disregard fields declared using
field(compare=False, hash=False|None).
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)
Thoughts?
On a different note, should the PEP be updated with the current signature? It still talks about hash=None being the
default.
--
~Ethan~
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