Newbie list question

[phawkins at connact.com]

>>>>> "MA" == Matthew Alton <Matthew.Alton at Anheuser-Busch.COM> writes:

MA> I am a UNIX/C programmer w/15y experience.  Forgive me if my neural
MA> pathways are all about following pointers.  The adjustment to
MA> Python-think is bumpy but I'll get by with a little help from my
MA> friends, eh?

MA> 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
MA> ['a', 'b', 'c']
>>>> bar 
MA> ['a', 'b', 'c']
>>>> baz
MA> ['a', 'b', 'c']            #  So far, so good.
>>>> del foo[2]             #  Get rid of 'c' in foo and, therefore in
MA> bar (?)
>>>> foo
MA> ['a', 'b']                 #  'c' is gone from foo...
>>>> bar
MA> ['a', 'b']                 #  ... and also from bar, as expected.
>>>> baz
MA> ['a', 'b', 'c']            #  baz, the copy, is unaffected.  Also as
MA> expected.
>>>> foo = foo + ['c']      #  Add 'c' back to foo.

You reassigned foo.  

foo + ['c'] creates a new object; bar now points to the original
object, foo points to a new and different object.  Foo is a label
bound to an object -- think key-value pair, not pointer to an address.

Try foo.append('c')
or foo += 'c'

>>>> foo
MA> ['a', 'b', 'c']            #  'c' is back.  Good.
>>>> bar
MA> ['a', 'b']                 #  ??? What the... ???  Where is 'c'?
>>>> baz
MA> ['a', 'b', 'c']            #  baz still unaffected, of course.
>>>> 

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

MA> Any help is appreciated.
MA> -- 
Patricia J. Hawkins
Hawkins Internet Applications, LLC