Marko Rauhamaa wrote:
Python <python@example.invalid>:
Marko Rauhamaa wrote:
Python <python@example.invalid>:
Marko Rauhamaa wrote:
id() is actually an ideal return value of __hash__(). The only criterion
is that the returned number should be different if the __eq__() is
False. That is definitely true for id().
$ python
Python 2.7.13 (default, Jan 19 2017, 14:48:08)
[GCC 6.3.0 20170118] on linux2
Type "help", "copyright", "credits" or "license" for more information.
nan = float('NaN')
id(nan) == id(nan)
True
nan == nan
False
Point being?
It is a counter example to your claim that if __eq__(...) is false
then id should return different values.
No it's not:
* __hash__() *should* return different values. It is neither possible
nor necessary in the general case.
* For NaN, there's no better alternative.
* Dictionaries and sets try "is" before __eq__(...) so everything works
anyway.
So, to be precise, the __hash__() rule is:
a.__hash__() *should* return a different number than b.__hash__() if
a is not b and not a.__eq__(b)
a.__hash__() *must* return the same number as b.__hash__() if
a is b or a.__eq__(b)
I didn't disagree with any of these statements about __hash__, but only
your statement about id and __eq__:
id() is actually an ideal return value of __hash__(). The only criterion
is that the returned number should be different if the __eq__() is
False. That is definitely true for id()
nan is a clear, simple, undeniable counterexample to that claim.
the hash function for floats is quite interesting btw, you may want to
look what is its value for nan.
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