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:

def delete_head(t): del t[0] letters = ['a', 'b', 'c'] delete_head(letters) print(letters)

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

Q-2: What would print when the following code executes?

def myfun(lst):
    del lst[0]

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

• ['a', 'b']
• myfun alters the state of the list object by removing the value at slot 0.
• ['b']
• myfun alters the state of the list object by removing the value at slot 0.

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:

t1 = [1, 2] t2 = t1.append(3) print(t1) print(t2) t3 = t1 + [3] print(t3) print(t2 is t3)

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.

Q-4: True or False. The following code block will not remove the first element from the list argument.

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

• True
• The slice operator creates a new list called "t", but that will not affect the list it was passed.
• False
• The slice operator creates a new list called "t", so it will not modify the original list.

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:

def tail(t): return t[1:] letters = ['a', 'b', 'c'] rest = tail(letters) print(rest)

Q-6: Which of the following list methods or operators will not create a new list when used.

• +
• Using the + operator will create a new list, not modify the original.
• append
• The append method modifies the original list, rather than creating a new one.
• slice
• The slice operator creates a new list, rather than modifying the original.

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