Think Python - Denison University

146 Chapter 15. Classesand objectsBecausePointisdefined at the toplevel, its“full name” is__main__.Point.The class object is like a factory for creating objects. To create a Point, you call Point as if it wereafunction.>>> blank = Point()>>> print blankThe return value is a reference to a Point object, which we assign to blank. Creating a new objectiscalled instantiation, and the object isan instance ofthe class.When you print an instance, Python tells you what class it belongs to and where it is stored inmemory (theprefix0xmeans that the following number isinhexadecimal).15.2 AttributesYou can assignvalues toaninstance usingdot notation:>>> blank.x = 3.0>>> blank.y = 4.0This syntax is similar to the syntax for selecting a variable from a module, such as math.pi orstring.whitespace. Inthiscase,though,weareassigningvaluestonamedelementsofanobject.These elements arecalled attributes.As a noun, “AT-trib-ute” is pronounced with emphasis on the first syllable, as opposed to “a-TRIBute,”whichisaverb.The following diagram shows the result of these assignments. A state diagram that shows an objectand itsattributes iscalled anobject diagram:blankPointxy3.04.0The variableblank refers to a Point object, which contains two attributes. Each attribute refers to afloating-point number.You can read the value of an attributeusingthe samesyntax:>>> print blank.y4.0>>> x = blank.x>>> print x3.0The expression blank.x means, “Go to the object blank refers to and get the value of x.” In thiscase,weassignthatvaluetoavariablenamedx. Thereisnoconflictbetweenthevariablexandtheattributex.You can use dot notation as part of any expression. For example: