Totally Confused: Passing variables to functions

[dique]

to see why: compare the following two cases

def blah(arg):
    arg.append(3)
 
v = [1,2]
 
blah(v)

then v becomes [1,2,3]



def blah(arg):
    arg = arg + [3]
 
v = [1,2]
 
blah(v)

then v still is [1,2]



Chuck <cdreward at riaa.com> wrote in message news:<m6ptdvg3p8dvcbpg7qhelb7s2i0slrk7b0 at 4ax.com>...
> I've hit a stumbling block while trying to pick up Python. I've googled
> around, and I don't think I'm the first to have this question, but I haven't
> been able to find an answer that explains things for me.
> 
> def blah(arg):
>     arg.append(3)
> 
> v = [1,2]
> 
> blah(v)
> 
> ... This gives me the impression that Python passes variables by reference (by
> a "pointer"), and the "arg" in blah is the same as "v".
> 
> But it's not, because if I say "arg = None" inside the function, "v" is
> unchanged.
> 
> Also, if I do something similar:
> 
> def blah(arg):
>     arg = arg + 1
> 
> v = 1
> 
> blah(v)
> 
> I get the impression Python passes variables by copy.
> 
> *Huh?*
> 
> It seems that if you pass a mutable variable, you can change it, but only by
> using it's methods, ie arg.append(), and NOT by doing an "arg = (new value)".
> 
> And if you pass an immutable variable, you can't change it at all.
> 
> This doesn't seem very intuitive, which leads me to believe I'm missing "the
> big picture".
> 
> In the python tutorial, it says:
> 
>    Actually, call by object reference would be a better description,
>    since if a mutable object is passed, the caller will see any changes
>    the callee makes to it (items inserted into a list).
> 
> But I don't understand what "object reference" means. I'm familiar with "pass
> by copy" and "by reference(pointer)" in the Pascal or C languages...
> 
> What's going on? (*grin*)
> 
> 
> Thanks!