[Python-Dev] Patch to use dict subclasses in eval(), exec (original) (raw)

M.-A. Lemburg mal@lemburg.com
Tue, 29 Oct 2002 08:57:01 +0100

Jeff Epler wrote:

Using dict subclasses in eval() and exec ----------------------------------------

With the attached patch, you can now use a dict subclass which implements getitem as the global or builtin namespace for eval() and exec. If you do not use the feature, the performance impact is low (<2%). The fast case adds only one test+branch to each evalcode() call (to set 'fastglobals' and 'slowbuiltins'), each lookup in globals (to check 'fastglobals'), and each lookup in builtins (to check 'slowbuiltins'). If you do use the feature, the performance impact is unfortunately quite substantial (400%). If you use subclasses of dict with eval()/exec, but do not define getitem, the performance penalty is modest (~10%).

If all you want is a way to define a default value for a failing dictionary lookup, wouldn't it be better to simply add this feature to the standard dictionary implementation ?

I don't think that many people will use this feature, yet everybody would pay for the performance hit.

-- Marc-Andre Lemburg CEO eGenix.com Software GmbH

eGenix.com -- Makers of the Python mx Extensions: mxDateTime,mxODBC,... Python Consulting: http://www.egenix.com/ Python Software: http://www.egenix.com/files/python/