Question about name scope

[Ian Kelly]

On Wed, Feb 1, 2012 at 11:47 AM, Mel Wilson <mwilson at the-wire.com> wrote:
> I guess they want local symbols in functions to be pre-compiled.  Similar to
> the way you can't usefully update the dict returned by locals().  Strangely,
> I notice that
>
> Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56)
> [GCC 4.4.3] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>>>> def f(x):
> ...   exec x
> ...   exec 'print a'
> ...
>>>> f('a=4')
> 4
>>>>
>
> works, but I really cannot explain why.

I am not a dev, but I believe it works because assigning to locals()
and assigning via exec are not the same thing.  The problem with
assigning to locals() is that you're fundamentally just setting a
value in a dictionary, and even though it happens to be the locals
dict for the stack frame, Python can't figure out that it should go
and update the value of the optimized local to match.  exec, on the
other hand, compiles and executes an actual STORE_NAME operation.  Of
course, if the particular local variable hasn't been optimized by the
compiler, then updating locals() works just fine (although you
probably should not rely on this):

>>> def f(x, y):
...     locals()[x] = y
...     print locals()[x]
...     exec 'print ' + x
...
>>> f('a', 42)
42
42

Another interesting thing to note is that the print in your example
doesn't even need to be in a second exec, which I believe works
because the presence of any exec statement disables global variable
optimizations for the function.  Compare:

>>> def f(x):
...     locals()['a'] = 4
...     print a
...
>>> f('pass')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in f
NameError: global name 'a' is not defined
>>> def f(x):
...     locals()['a'] = 4
...     print a
...     exec x
...
>>> f('pass')
4

And while we're on the subject, here's a nicely obscure syntax error:

>>> def f(x):
...   def g():
...     print x
...   exec x
...
  File "<stdin>", line 4
SyntaxError: unqualified exec is not allowed in function 'f' it
contains a nested function with free variables

Cheers,
Ian