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

Nick Coghlan ncoghlan at gmail.com
Wed May 26 15:43:35 CEST 2010

On 26/05/10 23:08, Michael Foord wrote:

On 26/05/2010 13:51, Nick Coghlan wrote:

On 26/05/10 19:48, Mark Dickinson wrote:

This is a long way from my area of expertise (I'm commenting here because it was me who sent Colin here in the first place), and it's not clear to me whether this is a bug, and if it is a bug, how it could be resolved. What would the impact be of having the compiler produce 'LOADNAME' rather than 'LOADGLOBAL' here?

exec with a single argument = module namespace exec with two arguments = class namespace Class namespaces are deliberately exempted from lexical scoping so that methods can't see class attributes, hence the example in the tracker issue works exactly as it would if the code was written as a class body. class C: y = 3 def execfunc(): print y execfunc() With this code, y would end up in C.dict rather than the module globals (at least, it would if it wasn't for the exception) and the call to execfunc fails with a NameError when attempting to find y. I know I've closed other bug reports that were based on the same misunderstanding, and I didn't understand it myself until Guido explained it to me a few years back, so suggestions for improving the exec documentation in this area would be appreciated. Your explanation here is very clear. Is this in the documentation?

It isn't actually - I think Thomas may be right that the current exec documentation was largely written prior to the introduction of lexical scoping. So adding something along these lines would probably be a good place to start.

Unfortunately, even my explanation above isn't 100% correct - it misses a subtle distinction where lexical scoping will pass through a class definition nested inside a function definition and see the function scope, but the use of strings for exec code means that the scopes of any containing functions are ignored completely.

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

More information about the Python-Dev mailing list