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.

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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, "lst" refers to that new list, the original list is not changed.
• False
• The slice operator creates a new list, "lst" refers to that new list, the original list is not changed.

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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