Hashability questions

[Ian Kelly]

On Tue, May 15, 2012 at 3:25 AM, Christian Heimes <lists at cheimes.de> wrote:
> Code explains more than words. I've created two examples that some issues.
>
> Mutable values break dicts as you won't be able to retrieve the same
> object again:

Sure, you'll get no argument from me on that.  I was more interested
in the other one.

> When you mess up __eq__ you'll get funny results and suddenly the
> insertion order does unexpected things to you:
>
>>>> class Example2(object):
> ...     def __init__(self, value):
> ...         self.value = value
> ...     def __hash__(self):
> ...         return hash(self.value)
> ...     def __eq__(self, other):
> ...         return hash(self) == hash(other)
> ...
>>>> d = {}
>>>> ob = Example2("egg")
>>>> d[ob] = True
>>>> d
> {<__main__.Example2 object at 0x7fab66cb7610>: True}
>>>> d["egg"] = True
>>>> d
> {<__main__.Example2 object at 0x7fab66cb7610>: True}
>>>> d2 = {}
>>>> d2["egg"] = True
>>>> d2[ob] = True
>>>> d2
> {'egg': True}

That's just how sets and dicts work with distinct objects that compare
equal.  I don't see any fundamental difference between that and this:

>>> d = {}
>>> d[42] = True
>>> d
{42: True}
>>> d[42.0] = True
>>> d
{42: True}
>>> d2 = {}
>>> d2[42.0] = True
>>> d2
{42.0: True}
>>> d2[42] = True
>>> d2
{42.0: True}

I wouldn't consider the hashing of ints and floats to be broken.