Why can a function modify some arguments as perceived by the caller, but not others?

Question

I\'m trying to understand Python\'s approach to variable scope. In this example, why is f() able to alter the value of x, as perceived within

Answer 1

If the functions are re-written with completely different variables and we call id on them, it then illustrates the point well. I didn't get this at first and read jfs' post with the great explanation, so I tried to understand/convince myself:

def f(y, z):
    y = 2
    z.append(4)
    print ('In f():             ', id(y), id(z))

def main():
    n = 1
    x = [0,1,2,3]
    print ('Before in main:', n, x,id(n),id(x))
    f(n, x)
    print ('After in main:', n, x,id(n),id(x))

main()
Before in main: 1 [0, 1, 2, 3]   94635800628352 139808499830024
In f():                          94635800628384 139808499830024
After in main: 1 [0, 1, 2, 3, 4] 94635800628352 139808499830024

z and x have the same id. Just different tags for the same underlying structure as the article says.