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

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.

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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 corner

nums = [2,3,5] nums[1] = 7

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id1

2 1 3 2 5

list

type unique identifier

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id1 nums

Classes are user-defined Types

Classes are how we add new types to Python

Example Classes

• Point3
• Card
• Rect
• Person

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id2

x 2 y 3 z 5

Point3

class name

Simple Class Definition

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

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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

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Description Invariant Short summary More detail Attribute list Attribute Name

0.. 999999999 means from 0 to 999999999, inclusive This 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)

id8 w

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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 in range 0..999999999, and boss either a Worker or None. self.lname = last_name self.ssn = ssn self.boss = boss w = Worker('Andersen', 1234, None) # this is the call to the constructor # the constructor calls __init__

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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)
• f 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

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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

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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]""" self.name = name self.NetID = NetID self.is_auditing = is_auditing

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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

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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 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

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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!

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Student id1 id1 p1

Student Jon Li name jl200 NetID True

is_auditing

id2 p2 id2

Student Jill Ba name jb110 NetID False

is_auditing

2

enrollment

Recall: Objects can have Methods

Function: 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

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id1

Student Jon Li name jl200 NetID True

is_auditing __init__(self, name, NetID, is_auditing)

Student 2

enrollment

Complete Class Definition

class <class-name>(): """Class specification""" <assignment statements> <function definitions>

keyword class

Beginning of a class definition Specification (similar to one for a function) to define methods to define class variables

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

__init__(self, name, NetID, is_auditing)

Student

enrollment 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 which instance of Student it is working with

class Student():

def __init__(self, name, NetID, is_auditing): self.name = name self.NetID = NetID self.is_auditing = is_auditing

Student.enrollment = Student.enrollment + 1

def greet(self): """Prints information about the Student 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")

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__init__(self, …) greet(self)

Student

enrollment

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

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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 folder print(s1.name) # finds attribute in class folder print(s1.enrollment) ß dangerous

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id1 id1 s1

Student Jon Li name jl200 NetID True

is_auditing __init__(self, name, NetID, is_auditing)

Student 1

enrollment

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

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What gets Printed? (Q)

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

C: 1 2 2 3 2 A: 1 2 3 3 3 B: 1 2 3 3 2 D: 1 1 3 3 3

What gets Printed? (A)

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import cs1110 s1 = cs1110.Student(“Jon Li”, “jl200”, True) print(s1.enrollment) s2 = cs1110.Student(“Jill Bo”, “jb20”, False) print(s2.enrollment) s2.enrollment = 3 print(s1.enrollment) print(s2.enrollment) print(cs1110.Student.enrollment) What?!?&%$?? Look at that in the Python Tutor.

C: 1 2 2 3 2 A: 1 2 3 3 3 B: 1 2 3 3 2 D: 1 1 3 3 3

Don’t forget self, Part 1

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s1 = Student(“Jon Li”, “jl200”, True) s1.greet() TypeError: greet() takes 0 positional arguments but 1 was given

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

class Student(): """Prints information about the Student 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

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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 the Student 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!

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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 Point Precondition: x,y,z are numbers""" self.x = x self.y = y self.z = z …

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