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Newbie list question

Matthew Alton Matthew.Alton at Anheuser-Busch.COM
Fri Jul 13 12:43:08 EDT 2001 
I am a UNIX/C programmer w/15y experience.  Forgive me if my neural
pathways are all about following pointers.  The adjustment to
Python-think is bumpy but I'll get by with a little help from my
friends, eh?

Here's the crux of the biscuit:

>>> foo = ['a', 'b', 'c']  #  We have a list named 'foo.'  Excellent.
>>> bar = foo              #  bar points to foo.  Or does it?
>>> baz = foo[:]           #  baz is a copy of foo.
>>> foo
['a', 'b', 'c']
>>> bar 
['a', 'b', 'c']
>>> baz
['a', 'b', 'c']            #  So far, so good.
>>> del foo[2]             #  Get rid of 'c' in foo and, therefore in
bar (?)
>>> foo
['a', 'b']                 #  'c' is gone from foo...
>>> bar
['a', 'b']                 #  ... and also from bar, as expected.
>>> baz
['a', 'b', 'c']            #  baz, the copy, is unaffected.  Also as
expected.
>>> foo = foo + ['c']      #  Add 'c' back to foo.
>>> foo
['a', 'b', 'c']            #  'c' is back.  Good.
>>> bar
['a', 'b']                 #  ??? What the... ???  Where is 'c'?
>>> baz
['a', 'b', 'c']            #  baz still unaffected, of course.
>>> 

I have verified this behavior on Python 1.5.1 (AIX 4.3.3) and on
Python 2.1
(Solaris Sparc 2.8).  From my arcane perspective, this behavior is
utterly inconsistent and confusing.  I strongly suspect that I simply
do not correctly grok the list structure, but so far I am unable to
turn up an explanation in the literature.

Any help is appreciated.
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