dynamic creation of global Identifier

[Alexander Eisenhuth]

Hello alltogether,

I've a little problem in creating a new identifier in the global namespace. The
following code creates a as local var in the namespace of init()

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(newIdentifi er, param):
exec "global %s" % newId
exec "%s = A(par)" % newId

init()
print a, b

but why doesent exec "global ... create a identifier in the global namespace.

The next thing I want to do is to create a identifier in a packages namespace
anyhow from where init() is called. How can I (or can I not) access from within
a function the namespace of the "package" where it is defined?

Thanks a lot
Alexander

[Theerasak Photha]

Re: dynamic creation of global Identifier

On 10/11/06, Alexander Eisenhuth <newsuser@staco m-software.dewrot e:

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.

You can frob the globals like so

import __builtin__
__builtin__.__d ict__['foo'] = 42

But by the time you get there, it is almost certainly time to refactor.

-- Theerasak

[Steve Holden]

Re: dynamic creation of global Identifier

Theerasak Photha wrote:

On 10/11/06, Alexander Eisenhuth <newsuser@staco m-software.dewrot e:
>
>

>>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@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(newIdentifi er, param):
exec """\
global %s
%s = A(par)""" % (newId, newId)

init()
print a, b



sholden@bigboy ~/Projects/Python
$ python test38.py
ctr of __main__.A with 1
ctr of __main__.A with 2
<__main__.A instance at 0x186c6d2c<__ma in__.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
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://holdenweb.blogspot.com
Recent Ramblings http://del.icio.us/steve.holden