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Fun with sys.modules

Duncan Grisby dgrisby at uk.research.att.com
Mon Jun 12 13:46:47 EDT 2000 
I'm attempting to make a module appear under two different names, and
I've come up with the following. It does what I want, but I'm a bit
worried by one of the side effects...

# a.py
print "a started"
answer = 42

-------

# b.py
print "b started"
answer = 1234

# Now the evil bit
import sys, a
sys.modules["b"] = sys.modules["a"]

# This is what worries me
print "sys is", sys
print "answer is", answer

print "b finished"

-------

$ python
Python 1.5.2 (#1, Feb  1 2000, 16:32:16) 
 [GCC egcs-2.91.66 19990314/Linux (egcs- on linux-i386
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> import b
b started
a started
sys is None
answer is None
b finished


What happened to the globals in the running module!?  Presumably the
entry in sys.modules was the only reference to module b, so it was
deleted while it was still running. Is this safe?  My guess is that
it's OK since it's basically the same situation as destructors which
run after a module has been deleted.

Any comments?  Am I totally crazy to be considering this?  If it is
safe at the moment, what are the chances that a future Python release
will behave differently?

Cheers,

Duncan.

-- 
 -- Duncan Grisby  \  Research Engineer  --
  -- AT&T Laboratories Cambridge          --
   -- http://www.uk.research.att.com/~dpg1 --
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