How safe is a set of floats?

[Peter Otten]

Paul McGuire wrote:

> Just to beat this into the ground, "test for equality" appears to be
> implemented as "test for equality of hashes".  So if you want to
> implement a class for the purposes of set membership, you must
> implement a suitable __hash__ method.  It is not sufficient to
> implement __cmp__ or __eq__, which I assumed "test for equality" would
> make use of.  Not having a __hash__ method in my original class caused
> my initial confusion.

As with dictionaries, only items with the same hash are considered for 
equality testing.
 
> So would you suggest that any class implemented in a general-purpose
> class library should implement __hash__, since one cannot anticipate
> when a user might want to insert class instances into a set?  (It
> certainly is not on my current checklist of methods to add to well-
> behaved classes.)

A meaningful implementation would also have to make sure that the attributes
used to calculate hash and equality don't change over time. 

No, I wouldn't bother because YAGNI.

Peter