Classes Announcements for This Lecture Assignments Lab this Week - PowerPoint PPT Presentation

Lecture 17 Classes

Announcements for This Lecture Assignments Lab this Week • A4 Thursday at midnight • More prelim exercises § Hopefully you are on Task 4 § This time for-loops § Minor extension for reasons § Also tables, dictionaries • Will post A5 on Thursday Exams § Written assignment like A2 § Needs material from Tues • Last week for regrades • Will post A6 on Nov 3. § Limit them to valid issues § Not due until November 20 • Getting closer to prelim 2 § Want to avoid exam crunch 10/29/19 Classes 2

Recall: Objects as Data in Folders • An object is like a manila folder Unique tab identifier • It contains other variables § Variables are called attributes id2 § Can change values of an attribute (with assignment statements) 2.0 x • It has a “tab” that identifies it 3.0 y § Unique number assigned by Python § Fixed for lifetime of the object 5.0 z 10/29/19 Classes 3

Recall: Classes are Types for Objects • Values must have a type • Classes are how we add new types to Python § An object is a value § A class is its type id2 Types Point3 • int Classes 2.0 x • float class name • Point3 • bool • RGB 3.0 y • str • Turtle • Window 5.0 z 10/29/19 Classes 4

Recall: Classes are Types for Objects • Values must have a type • Classes are how we add new types to Python § An object is a value § A class is its type id2 Types In Python3, type and class Point3 • int Classes are now both synonyms 2.0 x • float class name • Point3 • bool • RGB 3.0 y • str • Turtle • Window 5.0 z 10/29/19 Classes 5

Classes Have Folders Too Object Folders Class Folders • Separate for each instance • Data common to all instances id2 Point3 Point3 id3 x 2.0 Point3 ???? y 3.0 x 5.0 z 5.0 y 7.2 z -0.5 10/29/19 Classes 6

Goes inside a The Class Definition module, just like a function definition. class < class-name > (object): """Class specification""" < function definitions > < assignment statements > < any other statements also allowed> Example class Example(object): """The simplest possible class.""" pass 10/29/19 Classes 7

Goes inside a The Class Definition module, just like a function keyword class definition. Beginning of a class < class-name > (object): class definition Do not forget the colon! Specification """Class specification""" more on this later (similar to one for a function) < function definitions > to define …but not often used < assignment statements > methods < any other statements also allowed> to define Example attributes class Example(object): Python creates """The simplest possible class.""" after reading the pass class definition 10/29/19 Classes 8

Recall: Constructors • Function to create new instances id2 e id2 § Function name == class name Example § Created for you automatically • Calling the constructor: § Makes a new object folder Will come § Initializes attributes Example back to this § Returns the id of the folder • By default, takes no arguments § e = Example() 10/29/19 Classes 9

Instances and Attributes • Assignments add object attributes id2 e § <object>.<att> = <expression> id2 § Example : e.b = 42 Example • Assignments can add class attributes 42 b § <class>.<att> = <expression> § Example : Example.a = 29 • Objects can access class attributes Example § Example : print e.a § But assigning it creates object attribute 29 a § Example : e.a = 10 • Rule : check object first, then class 10/29/19 Classes 10

Instances and Attributes • Assignments add object attributes id2 e § <object>.<att> = <expression> id2 § Example : e.b = 42 Not how Example usually done • Assignments can add class attributes 42 b § <class>.<att> = <expression> § Example : Example.a = 29 • Objects can access class attributes Example § Example : print e.a § But assigning it creates object attribute 29 a § Example : e.a = 10 • Rule : check object first, then class 10/29/19 Classes 11

Instances and Attributes • Assignments add object attributes id2 e § <object>.<att> = <expression> id2 § Example : e.b = 42 Example • Assignments can add class attributes 42 b § <class>.<att> = <expression> 10 a § Example : Example.a = 29 • Objects can access class attributes Example § Example : print e.a § But assigning it creates object attribute 29 a § Example : e.a = 10 • Rule : check object first, then class 10/29/19 Classes 12

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 floats § RGB class: all attributes must be ints in 0..255 • Purpose of the class specification 10/29/19 Classes 13

The Class Specification class Worker(object): """A class representing a worker in a certain organization Instance has basic worker info, but no salary information. Attribute lname: The worker last name Invariant: lname is a string Attribute ssn: The Social Security number Invariant: ssn is an int in the range 0..999999999 Attribute boss: The worker's boss Invariant: boss is an instace of Worker, or None if no boss""" 10/29/19 Classes 14

The Class Specification class Worker(object): Short """A class representing a worker in a certain organization summary Instance has basic worker info, but no salary information. More detail Description Attribute lname: The worker last name Invariant: lname is a string Invariant Attribute ssn: The Social Security number Invariant: ssn is an int in the range 0..999999999 Attribute boss: The worker's boss Invariant: boss is an instace of Worker, or None if no boss""" 10/29/19 Classes 15

The Class Specification class Worker(object): """A class representing a worker in a certain organization Instance has basic worker info, but no salary information. Warning: New format this year. Attribute lname: The worker last name Old exams will be very different. Invariant: lname is a string Attribute ssn: The Social Security number Invariant: ssn is an int in the range 0..999999999 Attribute boss: The worker's boss Invariant: boss is an instace of Worker, or None if no boss""" 10/29/19 Classes 16

Recall: Objects can have Methods • Object before the name is an implicit argument • Example : distance >>> p = Point3(0,0,0) # First point >>> q = Point3(1,0,0) # Second point >>> r = Point3(0,0,1) # Third point >>> p.distance(r) # Distance between p, r 1.0 >>> q.distance(r) # Distance between q, r 1.4142135623730951 10/29/19 Classes 17

Method Definitions 1. class Point3(object): • Looks like a function def 2. """Class for points in 3d space § Indented inside class 3. Invariant: x is a float § First param is always self 4. Invariant y is a float § But otherwise the same 5. Invariant z is a float """ 6. def distance (self,q): • In a method call : 7. """Returns dist from self to q § One less argument in () 8. Precondition: q a Point3""" § Obj in front goes to self 9. assert type(q) == Point3 • Example : a.distance(b) 10. sqrdst = ((self.x-q.x)**2 + 11. (self.y-q.y)**2 + self q 12. (self.z-q.z)**2) 13. return math.sqrt(sqrdst) 10/29/19 Classes 18

Methods Calls • Example : a.distance(b) 1. class Point3(object): 2. """Class for points in 3d space a id2 b id3 3. Invariant: x is a float 4. Invariant y is a float id2 id3 5. Invariant z is a float """ Point3 Point3 x 1.0 x 0.0 def distance (self,q): 6. y 2.0 y 3.0 7. """Returns dist from self to q z 3.0 z -1.0 8. Precondition: q a Point3""" 9. assert type(q) == Point3 10. sqrdst = ((self.x-q.x)**2 + 11. (self.y-q.y)**2 + 12. (self.z-q.z)**2) 13. return math.sqrt(sqrdst) 10/29/19 Classes 19

Methods Calls • Example : a.distance(b) 1. class Point3(object): 2. """Class for points in 3d space a id2 b id3 3. Invariant: x is a float 4. Invariant y is a float id2 id3 5. Invariant z is a float """ Point3 Point3 x 1.0 x 0.0 6. def distance (self,q): y 2.0 y 3.0 7. """Returns dist from self to q z 3.0 z -1.0 8. Precondition: q a Point3""" 9. assert type(q) == Point3 distance 9 10. sqrdst = ((self.x-q.x)**2 + 11. (self.y-q.y)**2 + self id2 12. (self.z-q.z)**2) q id3 13. return math.sqrt(sqrdst) 10/29/19 Classes 20