Infinitely nested containers

[Steven D'Aprano]

random832 at fastmail.us wrote:

> On Fri, Nov 21, 2014, at 02:00, Marko Rauhamaa wrote:
>> Gill Shen <gillarete at gmail.com>:
>> 
>> > How is this [nesting] behavior implemented under the hood?
>> 
>> Pointers.
>> 
>> > And why is this allowed at all?
>> 
>> There's no reason not to.
> 
> There's no reason not to allow it with tuples, but you can't do it.
> Mainly because doing it in a single literal would require special
> syntax, whereas you can simply append to a list a reference to itself.

You can't append a list to itself in a single expression, you have to create
the list first.

> I think I tried on at least one python version and printing the tuple
> crashed with a recursion depth error, since it had no special protection
> for this case the way list printing does.


It works fine now (Python 3.3).

py> L = []
py> t = (L, None)
py> L.append(L)
py> L.append(t)  # For good measure.
py> print(t)
([[...], (...)], None)


but yes, in old versions of Python printing self-recursive containers may
break.



-- 
Steven