[Python-Dev] very slow compare of recursive objects

[Martin v. Löwis]

Tim Peters <tim.one@comcast.net> writes:

> > 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

Or, using Guido's example:

>>> L=[]
>>> t=(L,)
>>> L.append(t)
>>> t[0][0] is t
True


> It's not clear that this matters to the recursion-checker, though.  

It doesn't: You can't create a tuple that contains itself, except on
the C level (which I would declare a bug in the C code that does so).
So any cycle involving a tuple must involve a non-tuple object as
well.

Regards,
Martin