Hashability questions

[Chris Rebert]

On Sun, May 13, 2012 at 12:11 PM, Bob Grommes <bob.grommes at gmail.com> wrote:
> Noob alert: writing my first Python class library.
>
> I have a straightforward class called Utility that lives in Utility.py.
>
> I'm trying to get a handle on best practices for fleshing out a library.  As such, I've done the following for starters:
>
>  def __str__(self):
>    return str(type(self))
>
> #  def __eq__(self,other):
> #    return hash(self) == hash(other)
>
> The commented-out method is what I'm questioning.  As-is, I can do the following from my test harness:
>
> u = Utility()
> print(str(u))
> print(hash(u))
> u2 = Utility()
> print(hash(u2))
> print(hash(u) == hash(u2))
>
> However if I uncomment the above _eq_() implementation, I get the following output:
>
> <class 'Utility.Utility'>
> Traceback (most recent call last):
>  File "/Users/bob/PycharmProjects/BGC/Tests.py", line 7, in <module>
>    print(hash(u))
> TypeError: unhashable type: 'Utility'
>
> Process finished with exit code 1
>
> Obviously there is some sort of default implementation of __hash__() at work and my implementation of _eq_() has somehow broken it.  Can anyone explain what's going on?

See the 3rd, 5th, and 4th paragraphs of:
http://docs.python.org/dev/reference/datamodel.html#object.__hash__

Also, for future reference, it's advisable to state whether your
question concerns Python 2.x or Python 3.x.

Cheers,
Chris
--
http://chrisrebert.com