CS105: Introduction to Python

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 15.1 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.

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Figure 15.1: Object diagram.

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

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
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.

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