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

Colin H hawkett at gmail.com
Thu May 27 13:14:05 CEST 2010

I needed to make a small modification to the workaround - I wasn't able to delete from 'stuff', as the definitions in exec()'d code won't run - they're relying on that being present at runtime. In practice the overhead of doing this is quite noticeable if you run your code like this a lot, and build up a decent sized context (which I do). It will obviously depend on the usage scenario though.

def define_stuff(user_code):  context = {...}  stuff = {}  stuff.update(context)

 exec(user_code, stuff)

return_stuff = {} return_stuff.update(stuff)

del return_stuff['builtins']  for key in context:    if key in return_stuff and return_stuff[key] == context[key]:      del return_stuff[key]

 return return_stuff

On Thu, May 27, 2010 at 2:13 AM, Colin H <hawkett at gmail.com> wrote:

Of course :) - I need to pay more attention. Your workaround should do the trick. It would make sense if locals could be used for this purpose, but the workaround doesn't add so much overhead in most situations.  Thanks for the help, much appreciated,

Colin On Thu, May 27, 2010 at 2:05 AM, Guido van Rossum <guido at python.org> wrote: On Wed, May 26, 2010 at 5:53 PM, Colin H <hawkett at gmail.com> wrote:

Thanks for the possible workaround - unfortunately 'stuff' will contain a whole stack of things that are not in 'context', and were not defined in 'usercode' - things that python embeds - a (very small) selection -

{..., 'NameError': <type 'exceptions.NameError'>, 'BytesWarning': <type 'exceptions.BytesWarning'>, 'dict': <type 'dict'>, 'input': <function input at 0x10047a9b0>, 'oct': , 'bin': , ...} It makes sense why this happens of course, but upon return, the globals dict is very large, and finding the stuff you defined in your usercode amongst it is a very difficult task.  Avoiding this problem is the 'locals' use-case for me.  Cheers, No, if taken literally that doesn't make sense. Those are builtins. I think you are mistaken that each of those (e.g. NameError) is in stuff -- they are in stuff['builtins'] which represents the built-in namespace. You should remove that key from stuff as well. --Guido Colin On Thu, May 27, 2010 at 1:38 AM, Guido van Rossum <guido at python.org> wrote: This is not easy to fix. The best short-term work-around is probably a hack like this:

def definestuff(usercode):  context = {...}  stuff = {}  stuff.update(context)  exec(usercode, stuff)  for key in context:  if key in stuff and stuff[key] == context[key]:  del stuff[key]  return stuff -- --Guido van Rossum (python.org/~guido)

-- --Guido van Rossum (python.org/~guido)

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