Tuple question

[Alex Martelli]

Roy Smith <roy at panix.com> wrote:

> I asked:
> > How is a class instance's mutability any less of disqualifier for
> > key-ness than a list's mutability?
> 
> Benjamin Niemann <pink at odahoda.de> wrote:
> 
> > a = [1, 2, 3]
> > b = [1, 2, 3]
> > if a == b:
> >     print "List equality is based on content"
> 
> Tuple (and string) equality is based on content too.  So what?  I can
> give my data class an __eq__ method, and then my class instance equality
> would also based on content.

And your class's instances wouldn't then be hashable any more unless
they defined a __hash__ method -- have you tried?

 
> So, to restate my original question, why should my mutable, 
> content-based-eqality class instance be a valid dictionary key, when a
> list is not?  Which part of a list's behavior makes it inherently 
> unusable as a key?  I'm not asking about design philosophy, I'm asking
> about observable behavior.

This was discussed in detail in another thread about 2 days ago, I
believe.  That thread started by somebody asking why modules were
hashable (could be keys in a dictionary).


Alex