[Python-Dev] variable name resolution in exec is incorrect (original) (raw)

Scott Dial scott+python-dev at scottdial.com
Thu May 27 15:09:55 CEST 2010

On 5/27/2010 7:14 AM, Colin H wrote:

def definestuff(usercode): context = {...} stuff = {} stuff.update(context)

exec(usercode, stuff) returnstuff = {} returnstuff.update(stuff) del returnstuff['builtins'] for key in context: if key in returnstuff and returnstuff[key] == context[key]: del returnstuff[key] return returnstuff

I'm not sure your application, but I suspect you would benefit from using an identity check instead of an eq check. The equality check may be expensive (e.g., a large dictionary), and I don't think it actually is checking what you want -- if the user_code generates an eq-similar dictionary, wouldn't you still want that? The only reason I can see to use eq is if you are trying to detect user_code modifying an object passed in, which is something that wouldn't be addressed by your original complaint about exec (as in, modifying a global data structure).

Instead of:

if key in returnstuff and returnstuff[key] == context[key]:

Use:

if key in returnstuff and returnstuff[key] is context[key]:

-- Scott Dial scott at scottdial.com scodial at cs.indiana.edu

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