[Python-Dev] Immutability vs. hashability

[Chris Barker]

I don't seem to be getting my own messages back to reply to, but:

yes, of course, dataclasses won't hash if a field is mutable:


In [*58*]: @dataclasses.dataclass(hash=*True*, frozen=*True*)

    ...: *class* *Hash*:

    ...:     x: int  = 5

    ...:     l: list = dataclasses.field(default_factory=list)

    ...:

    ...:

    ...:

    ...:


In [*59*]: h = Hash()


In [*60*]: hash(h)

---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

<ipython-input-60-ad376ae4487e> in <module>()

----> 1 hash(h)


/Users/chris.barker/miniconda2/envs/py3/lib/python3.6/site-packages/dataclasses.py
in __hash__(self)


TypeError: unhashable type: 'list'


And the hash does depend on the values of the fields.So you can REALLY
ignore my previous note.

Again, sorry for the noise.

-CHB



On Mon, Feb 5, 2018 at 3:37 PM, Steven D'Aprano <steve at pearwood.info> wrote:

> On Sun, Feb 04, 2018 at 09:18:25PM -0800, Guido van Rossum wrote:
>
> > The way I think of it generally is that immutability is a property of
> > types, while hashability is a property of values.
>
> That's a great way to look at it, thanks.
>
>
> --
> Steve
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