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]

http://www.cs.cornell.edu/courses/cs1110/2018sp

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• Contains other variables

§ Variables are called attributes§ Can change attribute values

(w/ assignment statements)• Has a “tab” that identifies it

§ Unique number assigned by Python§ Fixed for lifetime of the object

• Has a type listed in the cornernums = [2,3,5]nums[1] = 7 3

id1

0 2 1 3 2 5

list

type

unique identifier

7

id1nums

Classes are user-defined Types

Classes are how we add new types to Python

Example Classes• Point3• Card• Rect• Person

4

id2

x 2

y 3

z 5

Point3

class name

Simple Class Definition

class <class-name>():

"""Class specification"""<function definitions>

5

class Worker():"""An instance is a worker in an organization.

Instance has basic worker info, but no salary information.

ATTRIBUTES:lname: Worker’s last name. [str]ssn: Social security no. [int in 0..999999999]boss: Worker's boss. [Worker, or None if no boss]

The Class Specification

6

Description

Invariant

Shortsummary

Moredetail

Attributelist

AttributeName

0.. 999999999 means from 0 to 999999999, inclusiveThis is handy notation, not Python.

Constructors

• Function to create new instances§ function name is the class name

§ Created for you automatically

• Calling the constructor:§ Makes a new object folder

§ Initializes attributes (see next slide)

§ Returns the id of the folder

w = Worker('Andersen', 1234, None)

id8w

7

id8

lname 'Andersen'

ssn

boss

1234

None

Worker

Special Method: __init__

def __init__(self, last_name, ssn, boss):"""Initializer: creates a Worker

Has last_name, ssn, and boss

Pre: last_name a string, ssn an int inrange 0..999999999, and boss either a Worker or None.self.lname = last_nameself.ssn = ssnself.boss = boss

w = Worker('Andersen', 1234, None) # this is the call to the constructor# the constructor calls __init__ 8

id8

lname 'Erik'

ssn

boss

1234

None

Worker

two underscores

use self to assign

attributes

ß called by the constructor

Evaluating a Constructor Expression

Worker('Erik', 1234, None)

1. Creates a new object (folder) of the class Worker on the heap§ Folder is initially empty

2. Executes the method __init__§ self = folder name = identifier§ Other arguments passed in order§ Executes commands in initializer

3. Returns folder name, the identifier

id8

lname 'Erik'

ssn

boss

1234

None

Worker

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 specification10

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 numbersis_auditing: whether student is auditing the class [bool]"""

self.name = nameself.NetID = NetIDself.is_auditing = is_auditing

11

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"

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():"""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 numbersis_auditing: whether student is auditing the class [bool]"""self.name = nameself.NetID = NetIDself.is_auditing = is_auditingStudent.enrollment = Student.enrollment + 1

13

Where does this variable live???

Classes Have Folders Too

Object Folders

• Separate for each instance• Example: 2 Student objects

Class Folders

• Data common to all instances

• Not just data!• Everything common to all

instances goes here!

14

Studentid1id1p1 StudentJon Linamejl200NetIDTrueis_auditing

id2p2

id2Student

Jill Banamejb110NetIDFalseis_auditing

2enrollment

Recall: Objects can have MethodsFunction: call with object as argument

<function-name>(<arguments>)len(my_list)

Method: function tied to the object<object-variable>.<function-call>my_list.count(7)

• Attributes live in object folder• Class Attributes live in class folder• Methods live in class folder

15

id1Student

Jon Linamejl200NetIDTrueis_auditing

__init__(self, name, NetID, is_auditing)

Student

2enrollment

Complete Class Definition

class <class-name>():

"""Class specification"""<assignment statements>

<function definitions>

keyword classBeginning of a class definition

Specification(similar to one for a function)

to define methods

to define class variables

16

class Student():"""Specification goes here.""”enrollment = 0def __init__(self, last_name, ssn, boss):

. . . <snip> . . .

__init__(self, name, NetID, is_auditing)

Student

0enrollment

Python creates after reading the class definition

Method Definitions• Looks like a function def

§ But indented inside class§ 1st parameter always self

Example: p1.greet() § Go to class folder for p1

(i.e., Student) that’s where greet is defined

§ Now greet is called with p1 as its first argument

§ This way, greet knows whichinstance of Student it isworking with

class Student():def __init__(self, name, NetID, is_auditing):

self.name = nameself.NetID = NetIDself.is_auditing = is_auditingStudent.enrollment = Student.enrollment + 1

def greet(self):"""Prints information about theStudent to the screen"""print("Hi! My name is "+ self.name)print(”My NetID is "+ self.NetID)if self.is_auditing:

print("I'm auditing the class")17

__init__(self, …)greet(self)

Student0enrollment

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

• ⟨object⟩.⟨name⟩ means § Go the folder for object

§ Find attribute/method name

§ If missing, check class folder§ If not in either, raise error

s1 = Student(“Jon Li”, “jl200”, True)# finds attribute in object folderprint(s1.name) # finds attribute in class folderprint(s1.enrollment) ß dangerous

19

id1id1s1 StudentJon Linamejl200NetIDTrueis_auditing

__init__(self, name, NetID, is_auditing)

Student

1enrollment

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 variables1.enrollment = 9 # creates new object

# variable called enrollmentBetter to refer to Class Variables using the Class Name

20

What gets Printed? (Q)

21

import cs1110

s1 = cs1110.Student(“Jon Li”, “jl200”, True)print(s1.enrollment)s2 = cs1110.Student(“Jill Bo”, “jb20”, False)print(s2.enrollment)s2.enrollment = 3print(s1.enrollment)print(s2.enrollment)print(cs1110.Student.enrollment)

C: 12232

A: 12333

B: 12332

D: 11333

What gets Printed? (A)

22

import cs1110

s1 = cs1110.Student(“Jon Li”, “jl200”, True)print(s1.enrollment)s2 = cs1110.Student(“Jill Bo”, “jb20”, False)print(s2.enrollment)s2.enrollment = 3print(s1.enrollment)print(s2.enrollment)print(cs1110.Student.enrollment)

What?!?&%$?? Look at that in the Python Tutor.

C: 12232

A: 12333

B: 12332

D: 11333

Don’t forget self, Part 1

23

s1 = Student(“Jon Li”, “jl200”, True)s1.greet()

TypeError: greet() takes 0positional arguments but 1 was given

<var>.<method_name> always passes <var> as first argument

class Student():

"""Prints information about theStudent to the screen"""def greet(self): # forgotten by you or me

print("Hi! My name is "+self.name)print(”My NetID is "+self.NetID)if self.is_auditing:

print("I'm auditing the class")

Don’t forget self, Part 2

24

s1 = Student(“Jon Li”, “jl200”, True)s1.greet()

NameError: global name ‘is_auditing' is not defined

<var>.<method_name> always passes <var> as first argument

class Student():

"""Prints information about theStudent to the screen"""def greet(self):

print("Hi! My name is "+self.name)print(”My NetID is "+self.NetID)if self.is_auditing: # forgot self!

print("I'm auditing the class")

Aside: The Value None

• The boss field is a problem.§ boss refers to a Worker object§ Some workers have no boss§ Or maybe not assigned yet

• Solution: use value None§ None: Lack of (folder) name§ Will reassign the field later!

25

id8

lname 'Erik'

ssn

boss

1234

None

Worker

Making Arguments Optional

• We can assign default values to __init__ arguments§ Write as assignments to

parameters in definition§ Parameters with default values

are optional• Examples:

§ p = Point3() # (0,0,0)§ p = Point3(1,2,3) # (1,2,3)§ p = Point3(1,2) # (1,2,0)§ p = Point3(y=3) # (0,3,0)§ p = Point3(1,z=2) # (1,0,2)

class Point3():"""Instances are points in 3d space

x: x coord [int]y: y coord [int]z: z coord [int] """

def __init__(self,x=0,y=0,z=0):"""Initializer: makes a new PointPrecondition: x,y,z are numbers"""self.x = xself.y = yself.z = z

…

26