Lecture 17: Classes (Chapter 15) CS 1110 Introduction to - PowerPoint PPT Presentation

http://www.cs.cornell.edu/courses/cs1110/2018sp Lecture 17: Classes (Chapter 15) CS 1110 Introduction to Computing Using Python [E. Andersen, A. Bracy, D. Gries, L. Lee, S. Marschner, C. Van Loan, W. White]

Announcements • Lab 9 is out (just 2 of the 4 problems) • IMPORTANT: A3 updates on website! • More recursion examples, see the demos: § http://www.cs.cornell.edu/courses/cs1110/2014sp/lectures/index.php § https://www.cs.cornell.edu/courses/cs1110/2016fa/lectures/10-18- 16/modules/morefun.py • More daily practice: download the demo code we post for each lecture; remove the contents, and try to reproduce what we did in class. Or try to modify it in new ways. 2

Recall: Objects as Data in Folders • An object is like a manila folder unique • Contains other variables identifier § Variables are called attributes type § Can change attribute values id1 nums (w/ assignment statements) id1 list • Has a “tab” that identifies it 2 0 § Unique number assigned by Python 7 3 1 § Fixed for lifetime of the object 5 2 • Has a type listed in the corner nums = [2,3,5] nums[1] = 7 3

Classes are user-defined Types Classes are how we add Example Classes new types to Python • Point3 class name • Card • Rect id2 • Person Point3 2 x 3 y 5 z 4

Simple Class Definition class < class-name > (): """Class specification""" < function definitions > 5

The Class Specification Short class Worker(): summary """An instance is a worker in an organization. More detail Instance has basic worker info, but no salary information. Attribute list Description ATTRIBUTES: Invariant lname: Worker’s last name. [str] ssn: Social security no. [int in 0..999999999] boss: Worker's boss. [Worker, or None if no boss] Attribute 0.. 999999999 means from 0 to 999999999, inclusive This is handy notation, not Python. Name 6

Constructors • Function to create new instances § function name is the class name § Created for you automatically id8 • Calling the constructor: id8 w Worker § Makes a new object folder lname 'Andersen' § Initializes attributes (see next slide) ssn 1234 § Returns the id of the folder boss None w = Worker('Andersen', 1234, None) 7

Special Method: __init__ two underscores ß called by the constructor def __init__(self, last_name, ssn, boss): """Initializer: creates a Worker Has last_name, ssn, and boss id8 Worker Pre: last_name a string, ssn an int in range 0..999999999, and boss either lname 'Erik' a Worker or None. ssn 1234 use self to self.lname = last_name assign boss None self.ssn = ssn attributes self.boss = boss w = Worker('Andersen', 1234, None) # this is the call to the constructor # the constructor calls __init__ 8

Evaluating a Constructor Expression Worker('Erik', 1234, None) 1. Creates a new object (folder) of the class Worker on the heap id8 § Folder is initially empty Worker 2. Executes the method __init__ § self = folder name = identifier lname 'Erik' § Other arguments passed in order ssn 1234 § Executes commands in initializer boss None 3. Returns folder name, the identifier 9

Invariants • Properties of an attribute that must be true • Works like a precondition: § If invariant satisfied, object works properly § If not satisfied, object is “corrupted” • Examples : § Point3 class: all attributes must be ints § RGB class: all attributes must be ints in 0..255 • Purpose of the class specification 10

Checking Invariants with an Assert class Student(): """Instance is a student taking python""" def __init__(self, name, NetID, is_auditing): """Initializer: instance with name, NetID, auditing status name: student's full name [str] NetID: student's NetID [str], 2-3 letters + 1-4 numbers is_auditing: whether student is auditing the class [bool]""" assert type(name) == str, "name should be type str" assert type(NetID) == str, "NetID should be type str" assert type(is_auditing) == bool, "is_auditing should be type bool" self.name = name self.NetID = NetID self.is_auditing = is_auditing 11

Class Attributes Class Attributes: Variables that belong to the Class • One variable for the whole Class • Shared by all object instances • Access by <Class Name>.<attribute-name> Why? • Some variables are relevant to every object instance of a class • Does not make sense to make them object attributes • Doesn’t make sense to make them global variables, either Example: Suppose we want to keep track of how many students are enrolled in CS 1110 12

Class Variables for CS1110 class Student(): Where does this variable live??? """Instance is a student taking python""” enrollment = 0 def __init__(self, name, NetID, is_auditing): """Initializer: instance with name, NetID, auditing status name: student's full name [str] NetID: student's NetID [str], 2-3 letters + 1-4 numbers is_auditing: whether student is auditing the class [bool]""" self.name = name self.NetID = NetID self.is_auditing = is_auditing Student.enrollment = Student.enrollment + 1 13

Classes Have Folders Too Object Folders Class Folders • Separate for each instance • Data common to all instances • Example: 2 Student objects Student id1 id1 p1 Student enrollment 2 id2 name Jon Li p2 NetID jl200 True is_auditing • Not just data! id2 • Everything common to all Student instances goes here! name Jill Ba NetID jb110 14 is_auditing False

Recall: Objects can have Methods Function: call with object as argument id1 Student <function-name>(<arguments>) name Jon Li len(my_list) NetID jl200 Method : function tied to the object True is_auditing <object-variable>.<function-call> my_list.count(7) • Attributes live in object folder Student • Class Attributes live in class folder enrollment 2 • Methods live in class folder __init__(self, name, NetID, is_auditing) 15

Complete Class Definition keyword class Beginning of a class definition class < class-name > (): Specification """Class specification""" (similar to one for a function) < assignment statements > Student to define < function definitions > enrollment 0 class variables __init__(self, name, NetID, is_auditing) class Student(): to define methods """Specification goes here.""” Python creates enrollment = 0 after reading the def __init__(self, last_name, ssn, boss): class definition 16 . . . <snip> . . .

Student Method Definitions enrollment 0 __init__(self, …) • Looks like a function def greet(self) § But indented inside class class Student(): § 1 st parameter always self def __init __(self, name, NetID, is_auditing): self.name = name Example: p1.greet() self.NetID = NetID self.is_auditing = is_auditing § Go to class folder for p1 Student.enrollment = Student.enrollment + 1 (i.e., Student) that’s where greet is defined def greet(self): """Prints information about the § Now greet is called with p1 Student to the screen""" as its first argument print("Hi! My name is "+ self.name) § This way, greet knows which print(”My NetID is "+ self.NetID) instance of Student it is if self.is_auditing: working with print("I'm auditing the class") 17

Class Gotchas… and how to avoid them Rules to live by: 1. Refer to Class Variables using the Class Name s1 = Student(“Jon Li”, “jl200”, True) print(“current enrollment = “+str( Student.enrollment )) 2. Don’t forget self 18

Name Resolution for Objects id1 id1 • ⟨ object ⟩ . ⟨ name ⟩ means s1 Student name Jon Li § Go the folder for object NetID jl200 § Find attribute/method name True is_auditing § If missing, check class folder § If not in either, raise error s1 = Student(“Jon Li”, “jl200”, True) Student # finds attribute in object folder enrollment 1 print(s1.name) __init__(self, name, # finds attribute in class folder NetID, is_auditing) print(s1.enrollment) ß dangerous 19

Accessing vs. Modifying Class Variables • Recall: you cannot assign to a global variable from inside a function call • Similarly: you cannot assign to a class attribute from “inside” an object variable s1 = Student(“Jon Li”, “jl200”, True) Student.enrollment = 2 # updates class variable s1.enrollment = 9 # creates new object # variable called enrollment Better to refer to Class Variables using the Class Name 20

What gets Printed? (Q) import cs1110 A: B: 1 1 s1 = cs1110.Student(“Jon Li”, “jl200”, True) 2 2 print(s1.enrollment) 3 3 s2 = cs1110.Student(“Jill Bo”, “jb20”, False) 3 3 print(s2.enrollment) 3 2 s2.enrollment = 3 print(s1.enrollment) C: D: print(s2.enrollment) print(cs1110.Student.enrollment) 1 1 2 1 2 3 3 3 2 3 21

What gets Printed? (A) import cs1110 A: B: 1 1 s1 = cs1110.Student(“Jon Li”, “jl200”, True) 2 2 print(s1.enrollment) 3 3 s2 = cs1110.Student(“Jill Bo”, “jb20”, False) 3 3 print(s2.enrollment) 3 2 s2.enrollment = 3 print(s1.enrollment) C: D: print(s2.enrollment) print(cs1110.Student.enrollment) 1 1 2 1 2 3 What?!?&%$?? 3 3 Look at that in the Python Tutor. 2 3 22