Avoiding referencing (was: Python sequences reference - problem?)

[Thorsten Kampe]

* Alex Martelli
> Just remembed that EVERYTHING works by reference, and what
> could possibly be the problem...?

The problem is that we newbies sometimes don't want referencing:

#v+
def part(seq, indices):
    """ chop seq by indices """
    partition = []
    
    for slice in indices:
        partition.append(seq[:slice])
        del seq[:slice]
    partition.append(seq)
    return partition
#v-

>>> foo = [11, 22, 33, 44, 55, 66, 77]
>>> part(foo, [2, 3])
[[11, 22], [33, 44, 55], [66, 77]]  # yes
>>> foo
[66, 77]                            # aaarrrghhh

Obviously I did not pass 'foo' "by value" to function 'part' but by 
reference and obviously not only local variable 'seq' was modified in 
the function but "outer" variable 'foo', too.

So how to deal with that?

1. "Just don't call function 'part' by with a variable, call it by 
value - part([11, 22, 33, 44, 55, 66, 77], [2, 3]) - and everything's 
okay".

Obviously no solution to me.

2. "If you don't want referencing, call it with a copy of 'foo' like: 
part(foo[:], [2, 3])"

Yes, but how do I know in advance that 'part' modifies 'foo'? I might 
not be the author of 'part'.

3. "Rewrite function 'part' so that it works with a tuple as argument. 
Thereby you're avoiding all possible modifying of existing objects."

Yes, but I'm also losing the "extended" possibilities of lists: 
"list.append", "list.reverse", "list.index" etc. My code will be twice 
as long as it would be with list methods.


Is there a way to "bypass" referencing _without_ disadvantages? I'm 
sure there is a "canonical" way to do this in Python!

Thorsten