import and shared global variables

Tim Hochberg tim.hochberg at ieee.org
Fri Mar 10 15:22:16 CET 2006


Michael Brenner wrote:
> Hi,
> 
> I'm implementing a plugin-based program, structured like the example 
> below (where m1 in the main module, loading m2 as a plugin).  I wanted 
> to use a single global variable (m1.glob in the example) to store some 
> config data that the plugins can access.  However, the output shown 
> belown seems to imply that glob is *copied* or recreated during the 
> import in m2.  Am I missing something? I thought m1 should be in 
> sys.modules and not be recreated during the import in m2.
> 
> After browsing c.l.p, it seems that this is probably somehow due to the 
> circular import.  However, I do not really see why this should be a 
> problem here.  Interestingly, the problem disappears when I put the code 
> in m1 in a real main() function instead of "if __name__" etc.  Though 
> this seems to solve my problem, I still want to understand what's 

What happens here is that there does end up being two copies of m1: the 
one named __main__ and the one imported as m1. If you think about this, 
there has to be two copies -- otherwise how could sometimes __name__ be 
__main__ and sometimes not.

Anyway, there are several options. The simplest one here is not to 
modify anything locally from __main__ block. Instead import m1, and 
modify that copy. That is:

glob = [1]
if __name__ == "__main__":
      import m1
      m1.glob.append(2)
      print "m1.main().1:", m1.glob
      m2 = __import__("m2")
      m2.test()
      print "m1.main().2:", glob


Regards,

-tim


> happening.
> 
> Thanks,
> 
> 	michael
> 
> 
> m1.py:
> ------
> glob = [1]
> if __name__ == "__main__":
>      glob.append(2)
>      print "m1.main().1:", glob
>      m2 = __import__("m2")
>      m2.test()
>      print "m1.main().2:", glob
> ------
> 
> m2.py:
> ------
> def test():
>      import m1
>      print "m2.test():", m1.glob
> -----
> 
> Output:
> m1.main().1: [1, 2]
> m2.test(): [1]
> m1.main().2: [1, 2]




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