[Python-Dev] Accessing globals without dict lookup

2002年2月11日 10:02:38 -0500

> Very nice. One case I would like to see covered is that of a global
> that is deleted. Something like:
>>> import eggs
>> i = -2
> max = 3
> j = 4
>> def foo(n):
> y = abs(i) + max
> return eggs.ham(y + n)
>> del j
>> I presume there would still be an entry in spam's module dict with a
> NULL objptr.

Yes.
> The whole think makes sense to me if it avoids the possible two
> PyDict_GetItem calls in the LOAD_GLOBAL opcode. As I understand it, if
> accessed inside a function, LOAD_GLOBAL could be implemented something like
> this:
>> case LOAD_GLOBAL:

Surely you meant LOAD_GLOBAL_CELL.
> cell = func_cells[oparg];
> if (cell.objptr) x = cell->objptr;
> else x = cell->cellptr->objptr;
> if (x == NULL) {
> ... error recovery ...
> break;
> }
> Py_INCREF(x);
> continue;
>> This looks a lot better to me (no complex function calls).

Here's my version:
 case LOAD_GLOBAL_CELL:
 cell = func_cells[oparg];
 x = cell->objptr;
 if (x == NULL) {
 x = cell->cellptr->objptr;
 if (x == NULL) {
 ... error recovery ...
 break;
 }
 }
 Py_INCREF(x);
 continue;
> What happens in the module's top-level code where there is
> presumably no func_cells array? Do we simply have two different
> opcodes, one for use at the global level and one for use in
> functions?

It could use LOAD_GLOBAL which should use PyMapping_GetItem on the
globals dict. Or maybe even LOAD_NAME which should do the same.
But we could also somehow create a func_cells array (hm, it would have
to be called differently then I suppose).
(I've added these to the FAQs in PEP 280 too.)
--Guido van Rossum (home page: http://www.python.org/~guido/)