[Python-Dev] another dict crasher

Michael Hudson mwh@python.net
02 Jun 2001 08:09:07 +0100

"Tim Peters" <tim.one@home.com> writes:

> [Michael Hudson]
> > Actually this crash was dict_print (I always forget about tp_print...).
> We all should <wink>.
> > It's pretty easy to mend:
> > I'm not sure this stops still more Machiavellian behaviour from
> > crashing the interpreter,
> Alas, it doesn't.

No, that's what my "dict[Machiavelli()] = Machiavelli()" example was

If noone beats me to it, I'll post a better fix to sf next week,
complete with test-cases and suitably "encouraging" comments.

I can't easily see other examples of the problem; there certainly
might be things you could do with comparisons that could trigger
crashes, but that code's so hairy that it's almost impossible for me
to be sure.

  There are two ways of constructing a software design: one way is to
  make it so simple that there  are obviously no deficiencies and the
  other way  is to make it so complicated  that there are  no obvious
  deficiencies.                                      -- C. A. R. Hoare

> > and you can certainly get items being printed more than once or not
> > at all.  I'm not sure this last is a problem;
> Those don't matter:  in a long tradition, we buy "safety" not only at the
> cost of bloating the code, but also by making the true behavior in case of
> mutation unpredictable & inexplicable.  

This is what I thought.

> > if the user's being this contrary there's only so much we can
> > do to help him or her.
> I'd prefer a similar internal immutable-dict trick that raised an exception
> if the user was pushing Python into a corner where "blow up or do something
> baffling" were its only choices.  That would render the original example
> illegal, of course.  But would that be a bad thing?  

It's hard to see how.

> What *should* it mean when the user invokes an operation on a
> container and mutates the container during that operation?

I don't think there's a meaning you can attach to this kind of

The "immutable dict trick" looks better the more I think about it, but
I guess that will have to wait until Guido gets back from the sun...


  incidentally, asking why things are "left out of the language" is
  a good sign that the asker is fairly clueless.
                                        -- Erik Naggum, comp.lang.lisp