[Python-ideas] __builtins__ behavior and... the FUTURE!

Jim Jewett jimjjewett at gmail.com
Sun Nov 25 02:21:37 CET 2007

On 11/24/07, Neil Toronto <ntoronto at cs.byu.edu> wrote:

[I'm summarizing and paraphrasing]

 If a name isn't in globals, python looks in
 Unfortunately, it may use a stale cached value for

> Well, keeping in mind this strange behavior and the length
> of time it's gone on, here's my recommendation:

>      Kill __builtins__. Take it out of the module dict. Let LOAD_GLOBAL
>      look in "builtins" (currently "__builtin__") for names after it
>      checks globals. If modules want to hack at builtins, they can
>      import it. But they hack it globally or not at all.

As Greg pointed out, this isn't so good for sandboxes.

But as long as you're changing dicts to be better namespaces, why not
go a step farther?  Instead of using a magic key name (some spelling
variant of builtin), make the fallback part of the dict itself.  For

Use a defaultdict and set the __missing__ method to the builtin's __getitem__.

Then neither python nor the frame need to worry about tracking the
builtin namespace, but the fallback can be reset (even on a
per-function basis) by simply replacing the fallback method.


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