[Python-Dev] violently deprecating exec without in (was: nested scopes. global: have I got it right?)

[Guido van Rossum]

>   >> >   SP> # top-level
>   >> >   SP> def g():
>   >> >   SP>   exec "x=3" return x
> 
>   GvR> [me]
>   >> Unfortunately this used to work, using a gross hack: when an exec
>   >> (or import *) was present inside a function, the namespace
>   >> semantics *for that function* was changed to the pre-0.9.1
>   >> semantics, where all names are looked up *at run time* first in
>   >> the locals then in the globals and then in the builtins.
>   >>
>   >> I don't know how common this is -- it's pretty fragile.  If
>   >> there's a great clamor, we can put this behavior back after b1 is
>   >> released.
> 
>   GvR> I spoke too soon.  It just works in the latest 2.1b1.  Or am I
>   GvR> missing something?
> 
> The nested scopes rules don't kick in until you've got one function
> nested in another.  The top-level namespace is treated differently
> that other function namespaces.  If a function is defined at the
> top-level then all its free variables are globals.  As a result, the
> old rules still apply.

This doesn't make sense.  If the free variables were truely considered
globals, the reference to x would raise a NameError, because the exec
doesn't define it at the global level -- it defines it at the local
level.  So apparently you are generating LOAD_NAME instead of
LOAD_GLOBAL for free variables in toplevel functions.  Oh well, this
does the job!

> Since class scopes are ignored for nesting, methods defined in
> top-level classes are handled the same way.
> 
> I'm not completely sure this makes sense, although it limits code
> breakage; most functions are defined at the top-level or in classes!
> I think it is fairly clear, though.

Yeah, it's pretty unlikely that there will be much code breakage of
this form:

def f():
    def g():
        exec "x = 1"
        return x

(Hm, trying this I see that it generates a warning, but with the wrong
filename.  I'll see if I can use symtable_warn() here.)

--Guido van Rossum (home page: http://www.python.org/~guido/)