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List:       python-ideas
Subject:    [Python-ideas] Re: classmethod __eq__ comparisons and Any wildcards
From:       Chris Angelico <rosuav () gmail ! com>
Date:       2021-09-14 13:49:05
Message-ID: CAPTjJmqOb7UDmA3x9YE_FSeN2Fgr_gnQoF5gOWWK+_mUwWEe-g () mail ! gmail ! com
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On Tue, Sep 14, 2021 at 11:30 PM <johnmelendowski@gmail.com> wrote:
> 
> I think I implemented __eq__ but not __hash__ and it broke the golden rule of x == \
> y => x is y i,.e. hash(x) == hash(y) 

Not sure what you're referring to there. The rule regarding hashes is
that if x == y, then hash(x) == hash(y). Identity doesn't come into
it, other than in the trivial sense that, for most classes, hash(x) ==
hash(x) (in other words, the hash is stable), and x == x, which
complies with the rule. But if x != x (eg with float("nan")), there's
no problem.

Incidentally, I don't think it's documented, but an object's hash
should never change. Otherwise, you can get bizarre behaviours:

> > > class X:
...     def __init__(self, value): self.thing = value
...     def __eq__(self, other): return self.thing == other.thing
...     def __hash__(self): return hash(self.thing)
...
> > > stuff = {X(5): "hello", X(7): "world"}
> > > stuff[X(5)]
'hello'

Well and good. But if we mutate the key...

> > > list(stuff)[0].thing = 2
> > > for key in stuff:
...     print(key, stuff[key])
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
KeyError: <__main__.X object at 0x7f1420409180>
> > > 

... we can iterate over the keys in the dictionary, use that exact
object to subscript the dictionary, and it isn't found. That key/value
pair has been placed in the bucket for 5, and won't be found in 2's
bucket.

If equality can change, __hash__ should be omitted.

ChrisA
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