htt://www.cs.cornell.edu/courses/cs1110/2018s Lecture 7: Objects (Chater 15) CS 1110 Introduction to Comuting Using Python [E. Andersen, A. Bracy, D. Gries, L. Lee, S. Marschner, C. Van Loan, W. White]
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Tye: set of values & oerations on them Tye float: Values: real numbers Os: +, -, *, /, ** Tye int: Values: integers Os: +, -, *, //, %, ** Tye bool: Values: integers Os: not, and, or Tye str: Values: string literals Double quotes: abc Single quotes: abc Os: + (concatenation) 7
Built-in Tyes are not Enough Want a oint in 3D sace We need three variables x, y, z coordinates What if have a lot of oints? Vars x0, y0, z0 for first oint Vars x1, y1, z1 for next oint This can get really messy How about a single variable that reresents a oint? x 2 y 3 z 5 8
Built-in Tyes are not Enough Want a oint in 3D sace We need three variables x, y, z coordinates What if have a lot of oints? Vars x0, y0, z0 for first oint Vars x1, y1, z1 for next oint This can get really messy How about a single variable that reresents a oint? Can we stick them together in a folder? Motivation for objects x 2 y 3 z 5 9
Objects: Organizing Data in Folders An object is like a manila folder It contains other variables Variables are called attributes These values can change It has an ID that identifies it Unique number assigned by Python (just like a NetID for a Cornellian) Cannot ever change Has no meaning; only identifies Unique tab identifier x 2 y 3 z 5 10
Classes: user-defined tyes Values must have a tye An object is a value Object tye is a class Modules rovide classes Examle: shaes.y Defines:, Rectangle classes x 2 y 3 z 5 class name 11
Constructor: Function to make Objects How do we create objects? Other tyes have literals No such thing for objects Constructor Function: Format: class name ( arguments ) Examle: (0,0,0) Makes a new object (manila folder) with a new id Called an instantiated object Returns folder id as value Examle: = (0, 0, 0) Creates a Point object x 0 y 0 z 0 Variable stores ID not object instantiated object Stores object s id in 12
Constructors and Modules >>> imort shaes >>> = shaes.(0,0,0) >>> id() Need to imort module that has Point class. Constructor is function. Prefix w/ module name. Shows the id of x 0 y 0 z 0 13
Accessing Attributes Attributes are variables that live inside of objects Can use in exressions Can assign values to them Format: variable. attribute Examle:.x Look like module variables To evaluate.x, Python: 1. finds folder with id stored in 2. returns the value of x in that folder id3 id3 x 1 y 2 z 3 14
Accessing Attributes Examle Examle: = shaes.(1, 2, 3).x =.x + 3 id3 id3 x 1 x 4 y 2 z 3 15
Object Variables Variable stores object id Reference to the object 1 2 Reason for folder analogy Assignment uses object id Examle: 1 = shaes.(0, 0, 0) 2 = 1 Takes contents from 1 Puts contents in 2 Does not make new folder! x 0 y 0 z 0 This is the cause of many mistakes in this course 16
Attribute Assignment (Question) >>> = shaes.(0,0,0) >>> q = Execute the assignments: >>>.x = 5 >>> q.x = 7 id4 id4 q id4 What is value of.x? x 0 A: 5 B: 7 C: id4 D: I don t know y 0 z 0 17
Attribute Assignment (Solution) >>> = shaes.(0,0,0) >>> q = Execute the assignments: >>>.x = 5 >>> q.x = 7 What is value of.x? A: 5 B: 7 CORRECT C: id4 D: I don t know id4 q id4 id4 x 0x 5 7 y 0 z 0 x 18
Call Frames and Objects (1) Objects can be altered in a function call Object variables hold ids! Folder can be accessed from global variable or arameter Examle: 1 def incr_x(q): q.x = q.x + 1 >>> = shaes.(1, 2, 3) >>> incr_x() Global Memory id5 id5 x 1 Call Frame incr_x 1 q id5 20
Call Frames and Objects (2) Objects can be altered in a function call Object variables hold ids! Folder can be accessed from global variable or arameter Examle: 1 def incr_x(q): q.x = q.x + 1 >>> = shaes.(1, 2, 3) >>> incr_x() Global Memory id5 id5 x x 1 2 Call Frame incr_x q id5 RETURN NONE 21
Call Frames and Objects (3) Objects can be altered in a function call Object variables hold ids! Folder can be accessed from global variable or arameter Examle: 1 def incr_x(q): q.x = q.x + 1 Global Memory id5 id5 x x 1 2 Call Frame >>> = shaes.(1, 2, 3) >>> incr_x() 22
How Many Folders (Question) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) Draw everything that gets created. How many folders get drawn? 23
How Many Folders (Solution) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) Draw everything that gets created. How many folders get drawn? 2/16/17 x 1 y 2 z 3 x 3 y 4 z 5 24
What Else? (Question) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) Draw everything that gets created. How many folders get drawn? What else gets drawn? 2/16/17 x 1 y 2 z 3 x 3 y 4 z 5 25
What Else? (Solution) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) Draw everything that gets created. How many folders get drawn? What else gets drawn? q 2/16/17 x 1 y 2 z 3 x 3 y 4 z 5 26
Swa (Question) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa_x(, q) Execute swa_x on what we just drew. There should be a call frame. What is in.x at the end? def swa_x(, q): 1 t =.x 2.x = q.x 3 q.x = t A: 1 B: 2 C: 3 D: I don t know 27
Swa (Solution) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa_x(, q) Execute swa_x on what we just drew. There should be a call frame. What is in.x at the end? def swa_x(, q): 1 t =.x 2.x = q.x 3 q.x = t A: 1 B: 2 C: 3 CORRECT D: I don t know 28
Swa Exlanation (1) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa_x(, q) def swa_x(, q): 1 t =.x 2.x = q.x 3 q.x = t q x 1 y 2 x 3 y 4 swa_x:1 1 q t z 3 z 5 29
Swa Exlanation (2) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa_x(, q) def swa_x(, q): 1 t =.x 2.x = q.x 3 q.x = t q x 1 y 2 x 3 y 4 swa_x:1 2 q t 1 z 3 z 5 30
Swa Exlanation (3) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa_x(, q) def swa_x(, q): 1 t =.x 2.x = q.x 3 q.x = t q x x 1 3 y 2 x 3 y 4 swa_x:1 3 q t 1 z 3 z 5 31
Swa Exlanation (4) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa_x(, q) def swa_x(, q): 1 t =.x 2.x = q.x 3 q.x = t q x x 1 3 y 2 z 3 x x 3 1 y 4 z 5 swa_x:1 q t 1 Return NONE 32
Swa Exlanation (5) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa_x(, q) def swa_x(, q): 1 t =.x 2.x = q.x 3 q.x = t q ERASE WHOLE FRAME x x 1 3 x x 3 1 y 2 y 4 z 3 z 5 33
Global (Question) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa(, q) Before calling swa(, q): q def swa(, q): 1 t = 2 = q 3 q = t What is in global after calling swa? A: B: C: I don t know 34
Global (Solution) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa(, q) After calling swa(, q): q def swa(, q): 1 t = 2 = q 3 q = t What is in global after calling swa? A: CORRECT B: C: I don t know 35
Global Exlanation (1) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa(, q) def swa(, q): 1 t = 2 = q 3 q = t q x 1 Point x 3 Point swa 1 q y 2 y 4 z 3 z 5 36
Global Exlanation (2) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa(, q) def swa(, q): 1 t = 2 = q 3 q = t q x 1 y 2 Point x 3 y 4 Point swa 2 q t z 3 z 5 37
Global Exlanation (3) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa(, q) def swa(, q): 1 t = 2 = q 3 q = t q x 1 y 2 Point x 3 y 4 Point swa t x q 3 z 3 z 5 38
Global Exlanation (4) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa(, q) def swa(, q): 1 t = 2 = q 3 q = t q x 1 y 2 z 3 Point x 3 y 4 z 5 Point swa t x RETURN q x NONE 39
Global Exlanation (5) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa(, q) def swa(, q): 1 t = 2 = q 3 q = t q Point Point ERASE WHOLE FRAME x 1 x 3 y 2 y 4 z 3 z 5 40
Global Revisited (Question) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa(, q) Before calling swa(, q): q def swa(a, b): 1 t = a 2 a = b 3 b = t What is in global after calling swa? A: B: C: I don t know 41
Global Revisited (Solution) imort shaes = shaes.(1,2,3) q = shaes.(3,4,5) swa(, q) After calling swa(, q): Same question as the revious one, (excet we re-named the arameters) but this time it seems obvious that global is unaffected. q def swa(a, b): 1 t = a 2 a = b 3 b = t What is in global after calling swa? A: CORRECT B: C: I don t know 42
Methods: Functions Tied to Classes Method: function tied to object Method call looks like a function call receded by a variable name: id3 Examle: variable. method ( arguments ) imort shaes = shaes.(1,2,3).greet() Hi! I am a 3-dimensional oint located at (4,2,3) id3 x 5 y 2 z 3 43
Built-in Tyes vs. Classes Built-in tyes Built-into Python Refer to instances as values Instantiate with literals Can ignore the folders Classes Provided by modules Refer to instances as objects Instantiate w/ constructors Must reresent with folders 44
Where To From Here? First, Understand objects All Python rograms use objects Most small rograms use objects of classes that are art of the Python Library Eventually, create your own classes: the heart of OO Programming the rimary tool for organizing Python rograms But we need to learn more basics first! 45