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1.12: List Arguments

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  • 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:

    def delete_head(t):
        del t[0]

    Here’s how it is used:

    >>> letters = ['a', 'b', 'c']
    >>> delete_head(letters)
    >>> print letters
    ['b', 'c']

    The parameter t and the variable letters are aliases for the same object. The stack diagram looks like Figure 10.12.1.

    Stack diagram.
    Figure \(\PageIndex{1}\): Stack diagram.

    Since the list is shared by two frames, I drew it between them.

    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
    [1, 2, 3]
    >>> print t2
    >>> t3 = t1 + [4]
    >>> print t3
    [1, 2, 3, 4]

    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.

    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:

    def tail(t):
        return t[1:]

    This function leaves the original list unmodified. Here’s how it is used:

    >>> letters = ['a', 'b', 'c']
    >>> rest = tail(letters)
    >>> print rest
    ['b', 'c']
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