There is an inconsistancy in the way that dictionary
subclasses behave
when they are used as as namespaces in execs.

Basically, while python 2.4 permits the usage of
dictionary subclasses
for local environments, it still bypasses the subclass
functions and
uses the C API for global environments. The attached
patch (and
unittest!) addresses this issue.

I'm pretty sure we keep the fast path in this.