Re: working with pointers

Michael wrote: 
if i do
a=2
b=a
b=0
then a is still 2!?

so when do = mean a reference to the same object and when does it mean make
a copy of the object??


It *always* means a reference. It *never* makes a copy.

Although the terminology isn't quite right, you can think of all "variables" in Python being "references". Assignment statements in Python then simply change the object that a "variable" "points" to.

Your example with integers:

py> a = 2
py> b = a
py> b = 0
py> a
2
py> b
0

A simlar example with lists:

py> a = [5, 7]
py> b = a
py> b = []
py> a
[5, 7]
py> b
[]

Of course, if you modify an object while two names are bound to it ("two variables hold pointers to it") then the modifications will be visible through either name ("either pointer"):

py> a = [5, 7]
py> b = a
py> a.pop()
7
py> a
[5]
py> b
[5]

Note that since integers are immutable, I can't give you a direct example like this with integers, but try:

py> class I(int):
....    pass
....
py> a = I(42)
py> a
42
py> b = a
py> b
42
py> a.flag = True
py> b.flag
True
py> a.flag = False
py> b.flag
False

So even with ints (or at least a mutable subclass of ints), modifications made to an object through one name ("reference") are also visible through other names ("references") to that object.

HTH,

STeVe
.