[Skip Montanaro]
> Just to see if I have a correct mental model of what Guido
> proposed, I drew a picture:
>
> http://manatee.mojam.com/~skip/python/celldict.png
>
> The cells are the small blank boxes. I guess the celldict would be the
> stuff I labelled "module dict". The "func cells" would be an array like
> fastlocals, but would refer to cells in the module's dict.
Yup, except that fastlocals are part of a frame, not part of a function
object. Guido didn't make a big deal about this, but it's key to
efficiency: the expense of setting up func_cells is *not* incurred on a
per-call basis, it's done once when a function object is created
(MAKE_FUNCTION), then reused across all calls to that function object.
> I'm not clear where/how builtins are accessed though.
__builtin__ is just another module, and also has a celldict for a __dict__.
The empty squares in your diagram (the "bottom half" of your cells)
sometimes point to cells in __builtin__'s celldict. They remain empty
(NULL) in __builtin__'s celldict, though.
> Is that what the extra indirection is, or are builtins incorporated into
> the module dict somehow?
In Guido's proposal, module celldicts sometimes point to builtin's cells.
It's set up so that *all* names of builtins get an entry in the module's
dict, even names that aren't referenced in the module (this avoids global
analysis). Their initial entries look like:
"len": {NULL, pointer to the "len" cell in the builtins}
Setting "len" as a module global (if you ever do that) overwrites the NULL.
Then later del'ing "len" again (if you ever do that) restores the NULL. For
*most* purposes, a cell with a NULL first pointer acts as if it didn't
exist. It's only the eval loop that understands the "deep structure".
In the variant I sketched today, there are no cross-dict pointers, and the
initial entries look like
"len": {the actual value of "len" from builtins, true}
instead. Then mutating the builtins requires reaching back into modules and
updating their timdicts. In return, access code is simpler+faster, and
there aren't semantic changes (compared to today) if the builtins mutate
*after* a module's dict is initially populated (Guido's scheme appears
vulnerable here in at least two ways).
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