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4/28/2020

1

Lecture22:

Subclasses&Inheritance

(Chapter18)

CS1110

IntroductiontoComputingUsingPython

[E.Andersen,A.Bracy,D.Fan,D.Gries,L.Lee,

S.Marschner,C.VanLoan,W.White]

Goal: Make a drawing app

2Rectangles,Stars,

Circles,andTriangles

havealotincommon, buttheyarealso differentinvery fundamentalways....Sharing Work

Problem:Redundant code.

(Any time you copy-and-paste code, you are likely doing something wrong.)

Solution: Create a parent class with shared code

Then, create subclassesof the parentclass

A subclass deals with specific details different from the parent class 3

Defining a Subclass

classShape(): """A shape located at x,y """ def__init__(self, x, y): ... defdraw(self): ... classCircle(Shape): """An instance is a circle.""" def__init__(self, x, y, radius): ... defdraw(self): ... classRectangle(Shape): """An in stance is a rectangle. """ def__init__(self, x, y, ht, len): ... defdraw(self): ... Shape

RectangleCircle

__init__(self,x,y) draw(self) Shape __init__(self,x,y, ht, len) draw(self)

Rectangle(Shape)

__init__(self,x,y, radius) draw(self)

Circle(Shape)

Superclass

Parent class

Base class

Subclass

Child class

Derived class

4

Extending Classes

class (): """Class specification""" class variables initializer (__init__) methodsSo far, classes have implicitly extended object

Class to extend

(may need module name: .) 5 object and the Subclass Hierarchy •Subclassing creates a hierarchyof classesEach class has its own super class or parent

Until objectat the "top"

•objecthas many features

Default operators:

__ init__, __str__,__eq__

Which of these need to be

replaced?

Super class

Super super class

built-in class 6

Example

object Shape

Rectangle

Square

4/28/2020

2 __init__: write new one, access parent's classShape(): """A shape @ location x,y """ def__init__(self, x, y): self.x = x self.y = y classCircle(Shape): """Instance is a Circle @ x,y with size radius""" def__init__(self, x, y, radius): super().__init__(x, y) self.radius= radius•Want to use the original version of the method?

New method = original+more

Don't repeat code from the original

•Call old method explicitly

Object Attributes can be Inherited

id3

Circle

y2 x1 radius4.0

Initialized in

Circle

initializer 8 classShape(): """ A shape @ location x,y """ def__init__(self,x,y): self.x = x self.y = y classCircle(Shape): """Instance is a Circle @ x,y with size radius""" def__init__(self, x, y, radius): super().__init__(x,y) self.radius= radius

Initialized in

Shape initializer c1 = Circle(1, 2, 4.0)c1 id3

Can override methods; can access parent's version

classShape(): """Instance is shape @ x,y""" def__init__(self,x,y): def__str__(self): return"Shape @ ("+str(self.x)+", "+str(self.y)+")" defdraw(self):... classCircle(Shape): """Instance is a Circle @ x,y with radius""" def__init__(self,x,y,radius): def__str__(self): return"Circle: Radius="+str(self.radius)+" "+super().__str__() defdraw(self):... __init__(self) __str__(self) __eq__(self) object __init__(self,x,y) __str__(self) Shape __init__(self,x,y,radius) __str__(self)

Circle

Understanding Method Overriding

__init__(self) __str__(self) __eq__(self) object __init__(self,x,y) __str__(self) __eq__(self) draw(self) Shape __init__(self,x,y,radius) __str__(self) __eq__(self) draw(self)

Circle

c1 = Circle(1,2,4.0) print(str(c1)) •Which __str__do we use?

Start at bottom class folder

Find first method with name

Use that definition

•Each subclass automatically inheritsmethods of parent. •New method definitions override those of parent. 11

Name Resolution Revisited

•To look up attribute/method name

1.Look first in instance (object folder)

2.Then look in the class (folder)

•Subclasses add two more rules:

3.Look in the superclass

4.Repeat 3. until reach object

Often called the Bottom-Up Rule

radius4.0 id3 y 2x1 c1id3

Circle

object __init__(self,x,y, radius) draw(self)

Circle(Shape)

__init__(self,x,y) draw(self)

Shape()

c1 = Circle(1,2,4.0) r = c1.radius c1.draw()

Q1: Name Resolution and Inheritance

classA(): deff(self): return self.g() defg(self): return10 classB(A): defg(self): return14 defh(self): return18 •Execute the following: >>> a= A() >>> b= B() •What is value of a.f()? 12 A: 10 B: 14 C: 5

D: ERROR

E: I don't know

4/28/2020

3

Q2: Name Resolution and Inheritance

•Execute the following: >>> a= A() >>> b= B() •What is value of b.f()? 14 A: 10 B: 14 C: 5

D: ERROR

E: I don't know

classA(): deff(self): return self.g() defg(self): return10 classB(A): defg(self): return14 defh(self): return18

Start next video:

Design choices for

method draw 16

Demo using Turtle Graphics

17 A turtle holds a pen and can draw as it walks! Follows simples commands: •setx, sety- set start coordinate •pendown, penup- control whether to draw when moving •forward •turn Part of the turtle module in Python (docs.python.org/3.7/library/turtle.html) •You don't need to know it •Just a demo to explain design choices of draw() in our classes Shape, Circle, Rectangle, Square

Who draws what?

18 classShape(): """Moves pen to correct location""" defdraw(self): turtle.penup() turtle.setx(self.x) turtle.sety(self.y) turtle.pendown() classCircle(Shape): """Draws Circle""" defdraw(self): super().draw() turtle.circle(self.radius)

Note: need to import the turtlemodule

which allows us to move a pen on a 2D grid and draw shapes.

No matter the shape, we

want to pick up the pen, move to the location of the shape, put the pen down.

But only the shape

subclasses know how to do the actual drawing.

Job for

Shape

Job for

subclasses

See shapes.py, draw_shapes.py

Start next video:

Class attributes

19

Class Variables can also be Inherited

classShape(): # inherits from object by default """Instance is shape @ x,y""" # Class Attribute tracks total numshapes

NUM_SHAPES = 0

classCircle(Shape): """Instance is a Circle @ x,y with radius""" # Class Attribute tracks total numcircles

NUM_CIRCLES = 0

. . . object

NUM_SHAPES

Shape(Circle)

NUM_CIRCLES

Circle

0 0 20

4/28/2020

4

Q3: Name Resolution and Inheritance

•Execute the following: >>> a= A() >>> b= B() •What is value of b.x? 21
A: 4 B: 3 C: 42

D: ERROR

E: I don't know

classA(): x = 3 # Class Variable y = 5 # Class Variable deff(self): return self.g() defg(self): return10 classB(A): y = 4 # Class Variable z = 42 # Class Variable defg(self): return14 defh(self): return18

Q4: Name Resolution and Inheritance

•Execute the following: >>> a= A() >>> b= B() •What is value of a.z? 23
A: 4 B: 3 C: 42

D: ERROR

E: I don't know

classA(): x = 3 # Class Variable y = 5 # Class Variable deff(self): return self.g() defg(self): return10 classB(A): y = 4 # Class Variable z = 42 # Class Variable defg(self): return14 defh(self): return18

Why override __eq__ ? Compare equality

classShape(): """Instance is shape @ x,y""" def__init__(self,x,y): def__eq__(self, other): """If position is the same, then equal as far as Shape knows""" returnself.x == other.xandself.y == other.y classCircle(Shape): """Instance is a Circle @ x,y with radius""" def__init__(self,x,y,radius): def__eq__(self, other): """If radii are equal, let super do the rest""" returnself.radius== other.radiusandsuper().__eq__(other) eqvs. is == compares equality is compares identity c1 = Circle(1, 1, 25) c2 = Circle(1, 1, 25) c3 = c2 c1 == c2 ? c1 is c2 ? c2 == c3 ? c2 is c3 ? c1id4 id4

Circle

radius25y 1x 1 id5

Circle

radius25y 1x 1 c2id5 c3 id5

The isinstanceFunction

isinstance(,)

True if 's class is same as or

a subclass of

False otherwise

Example:

c1 = Circle(1,2,4.0) isinstance(c1,Circle)is True isinstance(c1,Shape) is True isinstance(c1,object) is True isinstance(c1,str) is False •Generally preferable to type

Works with base types too!

28
c1id4 id4

Circle

object Shape

Circleradius4.0y

2x 1

Q5: isinstanceand Subclasses

>>> s1 = Rectangle(0,0,10,10) >>> isinstance(s1, Square) 29

A: True

B: False

C: Error

D: I don't know

s1id5 id5

Rectangle

object Shape

Rectangle

y2x 1

Square

4/28/2020

5

A5: isinstanceand Subclasses

>>> s1 = Rectangle(0,0,10,10) >>> isinstance(s1, Square) 30

A: True

B: False

C: Error

D: I don't knowobject

Rectangle

Shape "extends" or "is an instance of"

Square

"extends" or "is an instance of""extends" or "is an instance of"quotesdbs_dbs14.pdfusesText_20
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