Section 9.13 List arguments

When you pass a list to a function, the function gets a reference to the list. If the function modifies a list parameter, the caller sees the change. For example, delete_head removes the first element from a list and is used like this:

The parameter t and the variable letters are aliases for the same object.

def myfun(lst):
    del lst[0]

mylist = ['a', 'b']
myfun(mylist)
print(mylist)

It is important to distinguish between operations that modify lists and operations that create new lists. For example, the append method modifies a list, but the + operator creates a new list:

This difference is important when you write functions that are supposed to modify lists. For example, this function does not delete the head of a list:

def bad_delete_head(t):
    t = t[1:]              # WRONG!

The slice operator creates a new list and the assignment makes t refer to it, but none of that has any effect on the list that was passed as an argument.

def deleting_first(lst):
    lst = lst[1:]

An alternative is to write a function that creates and returns a new list. For example, tail returns all but the first element of a list and leaves the original list unmodified. Here’s how it is used: