[Python-Dev] very slow compare of recursive objects

[Tim Peters]

[Tim]
>Tuples can't be recursive,

[Guido]
> Oh yes they can be:
>
>    >>> L = []
>    >>> t = (L, L)
>    >>> L.append(L)
>    >>>

That's an example of a tuple *containing* a recursive structure; there's
still no way, starting at t, to get back to t.  I think mutation is required
for that.  Like

    L = [None]
    t = (L, L)
    L[0] = t

Then

    t[0]0] is t

and that's what I mean by "recursive tuple" (a tuple that can be reached
from itself).

It's not clear that this matters to the recursion-checker, though.  If it
indeed requires mutation to create a recursive tuple in the sense I mean it,
then nesting of tuples alone can't create one.  Stick a mutable container
type into the mix, and the recursion-checker will eventually find that.

I expect we want to continue exempting deeply nested tuples, since they come
up in real life (e.g., Python "parse trees" routinely exceed NESTING_LIMIT).