Python sequences reference - problem?

Michal Kaukic mike at frcatel.fri.utc.sk
Thu Sep 19 15:54:02 CEST 2002


Hi,
   
   we like Python - I use it (with Numeric and SciPy modules) 
for teaching of Numerical analysis and my colleague is doing 
some research in discrete optimization in Python. And he often
surprises me with non-conformal use of language :-)

   We are mathematicians, not programmers. I suppose, he believes 
that Python "thinks" like Mathematician. Yesterday, he was struggling 
with the code shown below (somewhat simplified):
    

L_L=[[1,2,3], [2,3,4], [3,4,5]]

def do_process(LL):
    n=len(LL[0])
    rec = [0]*n
    Res=[]
    
    for row in LL:
        rec[0] = row[0]
        # further code - modifying rec 
        # lots of conditional processing and such...
        # ...
        Res += [rec]
                
    return Res

---------------------------------------------

After calling do_process(L_L), the "expected" result should be

   [[1, 0, 0], [2, 0, 0], [3, 0, 0]]

but the true result is

   [[3, 0, 0], [3, 0, 0], [3, 0, 0]].


   Yes, this is fully in accordance with how the Python language should
behave:
       Res += [rec] inserts references to list object rec,
       which are further modified... (he should use copy(rec) instead).

   But there is nothing to make this behaviour clearly VISIBLE in code.
If I work with pointers in C/C++ I know and see they are pointers.
You can say - we also know that rec is list object and so be careful
with it.  Yes, but consider the complex code where the similar constructs
are very easy to overlook. And debugging of such code can be frustrating.

   My colleague was in state of despair and made thoughtful remarks
about FORTRAN and Python similarity. He believes that Python is corrupting 
his (computer) memory...

   So what is the point? I wrote this message in hope that there are
more people with similar experience. My question is - how to explain
to novice non-programmer users (maybe mathematically "infected")     
the quirks of Python sequence objects? Which methodology to use
in programs so we can clearly see we work with "pointers"?
Or maybe someone can propose the changes to the Language to overcome
this (psychological) barrier? I feel this as a serious obstacle
in using Python (my students experienced it too).

Thanks,
                  Mike






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