dynamic creation of global Identifier

Steve Holden steve at holdenweb.com
Wed Oct 11 12:19:28 CEST 2006


Theerasak Photha wrote:
> On 10/11/06, Alexander Eisenhuth <newsuser at stacom-software.de> wrote:
> 
> 
>>but why doesent exec "global ... create a identifier in the global namespace.
> 
> 
> I haven't had much use for exec, but it operates in its own, more or
> less cloistered namespace. It can't set globals among other things.
> 
Well that's not strictly true:

  >>> globals()
{'__builtins__': <module '__builtin__' (built-in)>, '__name__': 
'__main__', '__file__': '/c/Steve/.pythonrc', 'sys': <module 'sys' 
(built-in)>, '__doc__': None}
  >>> exec "NEW = 42"
  >>> globals()
{'__builtins__': <module '__builtin__' (built-in)>, '__file__': 
'/c/Steve/.pythonrc', 'sys': <module 'sys' (built-in)>, 'NEW': 42, 
'__name__': '__main__', '__doc__': None}

The problem was that the two exec statements were being treated 
separately because they use independent execution contexts. Make them 
the same, and bingo!

sholden at bigboy ~/Projects/Python
$ cat test38.py
class A:
         def __init__(self, v):
                 print "ctr of", self.__class__, "with", v
                 self._v = v

def init():
         newIdentifier =  ["a", "b"]
         param = [1,2]
         for newId, par in zip(newIdentifier, param):
                 exec """\
global %s
%s = A(par)""" % (newId, newId)

init()
print a, b



sholden at bigboy ~/Projects/Python
$ python test38.py
ctr of __main__.A with 1
ctr of __main__.A with 2
<__main__.A instance at 0x186c6d2c> <__main__.A instance at 0x186c6e0c>

However, none of this will necessarily get the OP further, since the 
interpretation of the global statement will be as "global to the module 
that it appears in".

regards
  Steve
-- 
Steve Holden       +44 150 684 7255  +1 800 494 3119
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