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6.2: Attributes

  • Page ID
    15447
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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.

    The following diagram shows the result of these assignments. A state diagram that shows an object and its attributes is called an object diagram; see Figure 15.2.1.

    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:

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

    The expression blank.x means, “Go to the object blank refers to and get the value of x.” In this case, 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:

    >>> print '(%g, %g)' % (blank.x, blank.y)
    (3.0, 4.0)
    >>> distance = math.sqrt(blank.x**2 + blank.y**2)
    >>> print distance
    5.0
    

    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.

    Exercise \(\PageIndex{1}\)

    Write a function called distance_between_points that takes two Points as arguments and returns the distance between them.


    This page titled 6.2: Attributes is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Allen B. Downey (Green Tea Press) .

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