except clause syntax question

[Ian Kelly]

On Tue, Jan 31, 2012 at 5:53 PM, Chris Angelico <rosuav at gmail.com> wrote:
> On Wed, Feb 1, 2012 at 9:03 AM, Duncan Booth
> <duncan.booth at invalid.invalid> wrote:
>> Abitrarily nested tuples of exceptions cannot contain loops so the code
>> simply needs to walk through the tuples until it finds a match.
>
> Is this absolutely guaranteed? The C API for CPython provides:
> (Py2) http://docs.python.org/c-api/tuple.html#PyTuple_SetItem
> (Py3) http://docs.python.org/dev/c-api/tuple.html#PyTuple_SetItem
>
> which doesn't have massive warnings on it saying "USE THIS ONLY TO
> INITIALIZE A TUPLE" (compare, for instance, _PyTuple_Resize which does
> carry a similar warning). Is the assumption then that we're all
> adults, and that mutating a tuple is like passing a null pointer to an
> API function (aka "loaded gun in proximity to foot")?

I don't know why the docs are written the way that they are, but if
you check the code, you can see that PyTuple_SetItem will raise a
SystemError if the reference count is anything other than 1.  So I
think that it is only meant to be used with similar caution and
restraint.

Cheers,
Ian