TypeError: unhashable type

[Jeremy Hylton]

"Mark Hammond" <mhammond at skippinet.com.au> writes:

> "Jeremy Hylton" <jeremy at beopen.com> wrote in message
> news:F8476B9163F138C3.8DBC9B4CCE38EBEA.9FFEEE60294594AF at lp.airnews.net...
> 
> > It seems that "unhashable type" is a pretty obscure error message.  If
> > you understand hashes, types, and dictionary implementation, it's
> > fine.  If not, you're left scratch your head.
> 
> For the casual user, this generally means "dictionary keys must be
> immutable".
> 
> > It would be helpful to produce an error message like:
> > "dictionary key must be hashable type, not <type>"
> 
> This message isnt a real lot better.  Often I _know_ what the type is -
> its just "unhashable" - which still doesnt clue the user in.  The word
> "hashable" is a bigger problem than not printing the type.

I was hoping the new error message would provide a few more clues by
mentioning "hashable" and "dictionary key."  If the documentation for
dictionaries defined hashable and explained its significant, then a
user would have an easier time figuring out what went wrong.

I agree that "hashable" is puzzling, but I think it is doubly so when
you don't know its related to dictionary keys.

Printing the type of the object is probably of secondary importance.

> I think the situation could still be improved without the type name.  But
> for the life of me I can't think how.
> 
> "dictionary keys must be immutable" isnt really any better - WTF is
> "immutable"??

Yeah, immutable isn't any better.  In fact, it's bad because you can
write a mutable instance that has __hash__ and __cmp__ methods and use
it as a dictionary key.

> "dictionary keys must be objects unable to be changed" - WTF - I can
> change strings!!?  Or maybe this is trying to tell me that once I add a
> key, it can't be removed?
> 
> Maybe something like:
> "dictionary keys must be immutable (such as tuples, strings or ints, but
> not objects such as lists)" - *gag*.
> 

The right goal is probably a slightly clearer error message and
documentation tied specifically to that error message.  This matches a
general strategy I use when some piece of software spits out an
obscure error message.  I type the exact message into Google,
DejaNews, or AltaVista and see what comes up.  More often than not, I
find someone has already answered my question.

Jeremy