[Python-Dev] Accessing globals without dict lookup

Tim Peters tim.one@comcast.net
Sun, 10 Feb 2002 16:51:59 -0500

[Skip Montanaro]
> I'm still working my way through this thread, so forgive me if
> this has been hashed out already.  It seems to me that the correct
> thing to do is to convert plain dicts to celldicts when creating
> functions.

There's the problem of object identity:  it's possible for exec'ed code to
mutate the original dict while the exec'ed code is running, and Guido gave
an example where that can matter.  I had originally suggested building a
celldict that *contained* the original dict, reflecting mutations from the
former to the latter as they happened.  Mutations in the other direction go
unnoticed, though.  If the binary layouts are compatible enough, it may
suffice to replace the dict's type pointer for the duration.  Even then, the
exec'ed code may get tripped up via testing (directly or indirectly) the
type of the original dict (I suppose it could lie about its type ...).

> Besides, where are functions going to get created that are outside
> of your (PyhonLabs) control?

They aren't, but eval and exec and execfile allow users to pass in plain
dicts to be used for locals and/or globals.