Is 'everything' a refrence or isn't it?

Claudio Grondi claudio.grondi at freenet.de
Wed Jan 4 21:31:38 CET 2006


KraftDiner wrote:
> I was under the assumption that everything in python was a refrence...
> 
> so if I code this:
> lst = [1,2,3]
> for i in lst:
>    if i==2:
>       i = 4
> print lst
> 
> I though the contents of lst would be modified.. (After reading that
> 'everything' is a refrence.)
> so it seems that in order to do this I need to code it like:
> 
> lst = [1,2,3]
> for i in range(len(lst)):
>    if lst[i] == 2:
>       lst[i]=4
> print lst
> 
> Have I misunderstood something?
> 
To give a short answer: yes.

And now into the details assuming, that your misunderstanding comes from 
programming in C before evaluating Python:

If you write
   i = 4
following happens:
an integer literal  4  gets into the scope of Python script as an object 
of type integer and can be from now on reached using the identifier  i . 
So  i  is in this context a _kind of_ pointer/reference to the value 
'4', but ... in Python sense not in C sense.

if you write
   for i in lst:
then the identifier  i  'points' to values in  lst, but else as in C 
where you can use a pointer to change the value it points to by 
assigning to *i or where you can use a reference to change a value it 
references, in Python you don't change a value of  i  because  i  is an 
identifier and therefore has no value in sense of C - it is only a name 
used to reach a value.

When you write within the  'for i in lst:' loop
   i = 4
you just use the identifier  i  as a 'pointer' to another Python object 
as it was pointing to before.

Let's consider a list.
The  'identifier'(i.e. construct) lst[i]  pointing to a list element 
behaves in Python like what you understand in C as a reference.

This is the main difference between the meaning of = assignment in 
Python and in C and it usually results in problems with understanding it 
when someone has programmed C and expects from Python to be like C.

The core of the trouble is probably different understanding of the word 
'reference', which used in different contexts (e.g. Python and C) means 
different things. Therefore as Scott David Daniels says
"There is a reason people talk about names and bindings."

Claudio



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