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15.2: Attributes

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  • You can assign values to an instance using dot notation:

    >>> blank.x = 3.0
    >>> blank.y = 4.0

    This syntax is similar to the syntax for selecting a variable from a module, such as math.pi or string.whitespace. In this case, though, we are assigning values to named elements of an object. These elements are called attributes.

    As a noun, “AT-trib-ute” is pronounced with emphasis on the first syllable, as opposed to “a-TRIB-ute”, which is a verb.

    Figure \(\PageIndex{1}\) is a state diagram that shows the result of these assignments. A state diagram that shows an object and its attributes is called an object diagram.

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

    The variable blank refers to a Point object, which contains two attributes. Each attribute refers to a floating-point number.

    You can read the value of an attribute using the same syntax:

    >>> blank.y
    >>> x = blank.x
    >>> x

    The expression blank.x means, “Go to the object blank refers to and get the value of x.” In the example, we assign that value to a variable named x. There is no conflict between the variable x and the attribute x.

    You can use dot notation as part of any expression. For example:

    >>> '(%g, %g)' % (blank.x, blank.y)
    '(3.0, 4.0)'
    >>> distance = math.sqrt(blank.x**2 + blank.y**2)
    >>> distance

    You can pass an instance as an argument in the usual way. For example:

    def print_point(p):
        print('(%g, %g)' % (p.x, p.y))

    print_point takes a point as an argument and displays it in mathematical notation. To invoke it, you can pass blank as an argument:

    >>> print_point(blank)
    (3.0, 4.0)

    Inside the function, p is an alias for blank, so if the function modifies p, blank changes.

    As an exercise, write a function called distance_between_points that takes two Points as arguments and returns the distance between them.

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