Dynamically declared shared constant/variable imported twice problem

Gabriel Rossetti gabriel.rossetti at arimaz.com
Fri May 1 15:47:23 CEST 2009

Peter Otten wrote:
> Gabriel Rossetti wrote:
>> I have three modules A, B, C;
>> A declares this globally :
>>   UpdateEvent, UPDATE_EVENT_ID = wx.lib.newevent.NewEvent()
>> Then they import stuff from each other:
>> - A imports a constant from module B (in "__main__")
>> - A imports a class and some constants from module C
>> - B imports a constant from module A
>> - C imports UPDATE_EVENT_ID from module A
>> What happens is that since A imports stuff from B which then in turn
>> loads something from A, UPDATE_EVENT_ID gets redefined and when A sends
>> an event to C, C doesn't do anything since it doesn't have the same
>> UPDATE_EVENT_ID. Does anyone have an idea on how to fix this? If it had
>> been a statically defined variable then that wouldn't be "bad", but
>> since it's dynamic it does pose problems. More globally, how cn I
>> prevent this even with static constants? I don't think it's that great
>> to redefine the stuff multiple times and I've already had this problem
>> in the past with a constant dict.
> Imported modules are cached, so this problem shouldn't occur unless you use 
> A.py as your main script. In this case it will be cached as "__main__", and 
> if you import it elsewhere a second copy will be executed and cached as "A".
> To fix it create a wrapper script that doesn't create any objects that will 
> be used elsewhere, e. g.
> #!/usr/bin/env python
> import A
> A.main()
> assuming the function A.main() contains the code to start your application 
> and invoke that instead of A.py
> Peter
Thank you Peter, I understand better now. I am indeed using A.py as my 
main script. I will create a launcher then, thank you.


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