Load a module twice ... or not ???

Gordon McMillan gmcm at hypernet.com
Tue Mar 21 23:00:23 CET 2000


With all due respect, Philip, your questions were answered as they 
were asked. If you don't care and aren't bothered, why did you ask?

> Gordon McMillan wrote:
> 
> > Philip Payne writes:
> >
> > > Can anyone by any chance give me a definitive explanation to the
> > > following (Python 1.5.2).
> > >
> > > In the book 'Internet Programming with Python', by Watters, van Rossum
> > > and Ahlstrom, in the section 'How to Load a Module Twice, and Why You
> > > Shouldn't', on page 199/200 (my edition, anyway), it states that you
> > > shouldn't  have module interrelationships that result in a module
> > > running as a main program and also being imported by another module in
> > > the same interpreter.
> >
> > Here's the simple example:
> > --module A
> > import B
> > a = 1
> > if __name__ == '__main__':
> >   global a
> >   a = 2
> >   B.doit()
> > --module B
> > import A
> > def doit():
> >   print A.a
> > ---------
> > If you run A, it will print "1".
> 
> Gordon, the example may be simple but with global variables you are really
> having to try quite hard. I don't write code that does anything like this. All
> I do is have a module A.py that might be:
>   def f1(): ...
>   def f2(): ...
>   class C1: ...
>   def main():
>      c = C1()
>      c.GO()
>   if  __name__ == '__main__' : main()
> that runs as a main program in the PyApache-embedded interpreter. Then I have
> another module B.py that imports A just in order to use the function f1()
> defined there. B.py might be:
>   import A
>   def f3(): ...
>   class C2: ... includes references to A.f1() ...
>   def main():
>      c = C2()
>      c.GO()
> B also runs as a main program in the same interpreter 'session' as A did,
> since I don't kill the interpreter between Apache requests, but I really don't
> see why I should have any problems. All the examples I've seen so far involve
> global variables in one form or another.  I don't use global variables except
> for performance reasons to cache database data between Apache requests and
> these globals are only set once. What mystified me in WvRA's book was the
> references to classes being defined twice. I mean, so what if they are???
> 
> >
> >
> > > Since I don't, in practice, encounter any problems doing what WvRA say I
> > > shouldn't do, is this in fact an issue? Is the above quote relevant to
> > > current Python versions?? And if so, can someone tell me in detail under
> > > what circumstances there might be a problem?
> >
> > Yes (you're lucky); yes; see above.
> >
> > - Gordon
> 
> -- 
> http://www.python.org/mailman/listinfo/python-list



- Gordon




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