Function parameter scope

[Steven D'Aprano]

On Thu, 29 Jul 2010 07:30:34 +0530, Navkirat Singh wrote:

> Hi,
> 
> I had another question:
> 
> What is the scope of  a parameter passed to a function? I know its a
> very basic question, but I am just sharpening my basics :)
> 
> def func_something(x)
> 	return print(x+1);
> 
> Does x become a local variable or does it stay as a module scoped
> variable?
> 
> Though I think its local, but just want to be sure.


Yes, x is local.

However, be careful that Python does not make a copy of arguments passed 
to functions. So the local name refers to the same underlying object as 
the global name, and *modifications* to the *object* will be seen 
everywhere. But *reassignments* to the *name* are only seen locally.



def test(local_name):
    print(local_name)
    local_name = 2
    print(local_name)
    print(global_name)

global_name = 1
test(global_name)

=> prints 1, 2, 1.



def test(local_name):
    print(local_name)
    local_name.append(2)
    print(local_name)
    print(global_name)

global_name = [1]
test(global_name)

=> prints [1], [1, 2], [1, 2].


-- 
Steven