Problem with exec

[Justus Schwabedal]

Dear python users!
I try to setted up compile-free parallelism using the exec command.  
However I had some problems with namespaces which I find mysterious  
although I managed to work around. But the workaround is not nice, so  
I wonder if there are ways.
I do the following,

bash-3.2$ cat execBug.py
#! /usr/bin/python
header="""
from scipy import randn
def f():
         return randn()
"""
def g(head):
         exec header
         return f()
print "g(header) =",g(header)

bash-3.2$ ./execBug.py
g(header) =
Traceback (most recent call last):
   File "./execBug.py", line 10, in <module>
     print "g(header) =",g(header)
   File "./execBug.py", line 9, in g
     return f()
   File "<string>", line 4, in f
NameError: global name 'randn' is not defined

However this works:

bash-3.2$ cat execBug.py
#! /usr/bin/python
header="""
from scipy import randn
def f():
         return randn()
"""
exec header
print "f() =",f()

bash-3.2$ ./execBug.py
f() = 1.44707148916

If for the first example I add "global randn", it works again.
However I do not understand how the nested namespaces interact here.
Isn't this an unwanted feature in the functionality?
I'm looking foreward to your comments, Justus