Modifying lists in functions

Here is one consequence of the fact that we can change the contents of lists: we can change the contents of arguments to functions:

In [1]:
def change_list(L):
    L[0] = 5
    
if __name__ == '__main__':
    L1 = [1, 2, 3]
    change_list(L1)
    print(L1)

[5, 2, 3]

What happenned there is that the parameter L became an alias of the list L1. Therefore, when the contents of L changed, the contents of L1 changed, too.

Note that all that's going on there is that we can change the contents of lists. With both lists and integers, if we make a local variable refer to a new object, the variable which we used as an argument is unaffected:

In [2]:
def dont_change_list(L):
    L = [5]
    
def dont_change_integer(n):
    n = 5
    
if __name__ == '__main__':
    L1 = [7]
    dont_change_list(L1)
    print(L1) #[7]
    
    m = 7
    dont_change_integer(m)
    print(m)  #7

[7]
7