Scope (?) question

[MRAB]

Peter wrote:
> I am puzzled by what appears to be a scope issue - obviously I have
> something wrong :-)
> 
> Why does this work:
> 
> if __name__ == 'main':
>   execfile('test-data.py')
>   print data
> 
> and yet this doesn't (I get "NameError: global name 'data' not
> defined"):
> 
> def X():
>   execfile('test-data.py')
>   print data
> 
> where test-data.py is:
> 
> data = [1,2,3,4]
> 
> I checked help on execfile and could only find the following
> (mystifying) sentence:
> 
> "execfile() cannot be used reliably to modify a function’s locals."
> 
In the first example 'execfile' is passed globals() by default, which
'test-data.py' modifies.

In the second example 'execfile' is passed locals() by default (because
it's called from within a function), which 'test-data.py' modifies.

However, Python (well, CPython at least), optimises access to locals
within functions, which unfortunately means that changes to locals()
won't actually affect the function's locals.

Here's an example which tries to change the locals:

 >>> def test():
...     x = 0
...     print "Old value of x:", x
...     locals()["x"] = 1
...     print "New value of x:", x
...
 >>> test()
Old value of x: 0
New value of x: 0


Compare this with changing globals:

 >>> x = 0
 >>> print "Old value of x:", x
Old value of x: 0
 >>> globals()["x"] = 1
 >>> print "New value of x:", x
New value of x: 1