CS108 Lecture 16: User Defined Classes. Overview/Questions

Transcription

1 CS108 Lecture 16: User Defined Classes Aaron Stevens 23 February Overview/Questions Review: the function of functions Thinking about program structure Review: objects and classes How do we design a program using objects? How do we define/implement classes? 2

2 Functions: what s the point? Recall that a function is like a subprogram a program within a program. Function A sequence of statements which are given a name. This sequence can then be invoked by calling the name. 3 Function Declaration Syntax def function(<formal-parameter-list>): <statement> <statement> The keyword def indicates the identifier that follows it is a new function name. The <formal-parameter-list> is a list of 0 or more parameters required to invoke the function. The parentheses are a required part of the definition, as is the colon after the parentheses. 4

3 Variable Scope Scope refers to the places in a program where a variable name can be referenced. The variables in a function are local to that function. To use a variable from outside the function it must be passed in as a parameter. To get a value out from a function, the function must return a value. 5 Object Orientation Object Sort of an active data type, which supports both data and operations. Objects know stuff (they contain data) and also do stuff (they have operations) Method In OO parlance, objects interact by sending each other messages, also known as methods. A method is a request for an object to perform one of its operations. 6

4 Review: objects and classes Every object is an instance of a class, and the class describes the properties (data and behaviors) the object will have. So the first step in object-oriented program design is to think about the classes in a program. 7 Provider-side vs. Client-side Clients-side code uses objects defined by class definitions. Previously, we were writing code which was a client of some class definitions (graphics). Class definitions are provider coder: the author is providing a building block to be used later by a client. Now, we need to put our provider hats on 8

5 What does a class know? Encapsulation is the process of enclosing programming elements inside larger, more abstract entities. Consider a class Die (e.g. in a game of chance). What does the Die class need to know? What number is currently displayed How many sides it has 9 What does a class do? What does the Die class need to do? initialize data members roll itself -- pick a random value provide value when asked 10

6 Describing a Class: UML The Uniform Modeling Language is a diagramming tool which we can use to describe software design. A UML Class Diagram describes a class: Class name Data attributes (what it knows) Methods (what it does) 11 Writing a Class Definition A class is defined by a class statement: class <class-name>: <method-definition> <method-definition> <method-definition> Within the scope of the class are some/several method definitions, each of which is similar to a function definition. 12

7 The self reference variable Class method are defined like functions. Each method has an special first parameter, called self. self is a reference to the object on which the method is acting. self provides context for the action, and is used to reference the object s instance variables. 13 The Constructor The special method init is the object constructor. Python calls this special method to initialize a new object. Its job is to: to set initial data values. to do any start-up work before the object can be used. 14

8 The Constructor Continue the Die class: Add constructor: initialize data members What are the data members? How will they be initialized? General form for Constructor Definition: def init (self, <formal-parameters>): 15 Object Methods Methods provide the actions of an object, e.g. what the class does. Methods will: Carry out the object s actions Modify/store data into the object Get/reveal data from the object Provide a printable version of the object Provide a deep copy of the object 16

9 Object Methods Continue the Die class: Add methods: what does the class do? What data are required for these operations? Are these data stuff the object knows? Methods can use both data members and data passed in as formal parameters. Methods can return values or not. General form for Method Definition: def <method-name>(self, <formal-parameters>): 17 Testing a User-Defined Class Testing is crucial. Unit test code for a class is done in the same file as the class definition, for example in a main function. 18

10 The str method The str method is a special method used to obtain a string representation of an object. This is important should the client want to print out your object. General form for the str method: def str (self, <formal-parameters>): return <string> 19 Class Definition: Example A class is defined by a class statement: 20

11 Take-Away Points Objects Classes Data members Methods Constructor The variable self 21 Student To Dos HW06 due Tuesday 2/24 QUIZ 3 is on FRIDAY 2/27 Covers material in lectures (10-151) Objects, graphics, functions, if/elif/else, while Reading: Zelle chapter 10 (this week) 22