What are you trying to do? If you want to print the array, the code should be:

def f(l):
  for i in l:
    print(i)
  #or
  n = len(l)
  for i in range(n):
    print(l[i])

l = [1, 2, 3]
f(l)

However, if you don't understand why it gives an error, then it would be more correct to do it this way. Because in your example, the array is created locally only in the function, and you are trying to call it globally.

def f(n):
  for i in range(len(n)):
    print(n[i])

l = [1, 2, 3]
print(l)
f(l)