Pass by reference?

[Remco Gerlich]

Dale Strickland-Clark wrote in comp.lang.python:
> Also the following program proves to me that arguments are passed by value
> and not by reference:
> 
> def wibble(var):
>     var = 1
> 
> x = 0
> wibble(x)
> print x
> 
> It prints 0 showing that the assignment in wibble is made to a copy of x

A key thing about Python. All arguments are passed by reference. But
*assignment* is by reference too!

x refers to the integer 0.
wibble is called, now the local var 'var' also refers to the integer 0.
after 'var = 1', 'var' refers to the integer 1. x is unchanged.

Variables are just names for objects. Assignment gives that name to another
object. Assignment can only *change* an object if you assign to something
'inside' the object, like 'x[3] = 4' or 'ob.value = 3'. Integers are
immutable, can't be changed. Just like strings and tuples. Lists can be
changed.

Compare:

x = []
def app(list, value):
   list = list+[value]
   return list
app(x, 3)
print x

With:

x = []
def app(list, value):
   list.append(value)
   return list
app(x, 3)
print x


-- 
Remco Gerlich,  scarblac at pino.selwerd.nl

   This is no way to be
     Man ought to be free      -- Ted Bundy
       That man should be me