Dynamically declared shared constant/variable imported twice problem

Discussion in 'Python' started by Gabriel Rossetti, May 1, 2009.

  1. Hello everyone,

    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.

    Thank you,
    Gabriel
     
    Gabriel Rossetti, May 1, 2009
    #1
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  2. Gabriel Rossetti

    Peter Otten Guest

    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
     
    Peter Otten, May 1, 2009
    #2
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  3. 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.

    Gabriel
     
    Gabriel Rossetti, May 1, 2009
    #3
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