08-classes-objects.ppt

8/17/2019 08-classes-objects.ppt

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Unit 8

Classes and Objects; Inheritance

Special thanks to Roy McElmurry, John Kurkoski, Scott Shacro!t, Ryan "ucker, #aul $eck !ortheir ork%

E&cept here otherise noted, this ork is licensed under'http'((creati)ecommons%or*(licenses(by+nc+sa(%-


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2

OOP, Defning a Class

. #ython as built as a procedural lan*ua*e/ OO# e&ists and orks 0ne, but !eels a bit more 1tacked

on1

/ Ja)a probably does classes better than #ython 2*asp3

. 4eclarin* a class'

class name: statements


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3

Fields

name = value

/ E&ample'

class Point: x = 0

y = 0# mainp1 = Point()p1.x = 2p1.y = -5

/ can be declared directly inside class 2as shon here3or in constructors 2more common3

/ #ython does not really ha)e encapsulation or pri)ate 0elds. relies on caller to 1be nice1 and not mess ith objects5 contents

point.py

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3

class Point: x = 0

y = 0


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4

Using a Class

import class/ client pro*rams must import the classes they use

point_main.py

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10

from Point import *

# mainp1 = Point()p1.x = 7p1.y = -3...

# Python objects are dynamic can add fie!ds any time"p1.name = "Tyler Dr!en"


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5

Object Methods

!e name(sel$ parameter$ ...$ parameter): statements

# sel must be the 0rst parameter to any object method. represents the 1implicit parameter1 2t$is in Ja)a3

/ must access the object5s 0elds throu*h the sel re!erence

class Point:

def trans!atese!f$ d%$ dy& se!f.x %= !x se!f.y %= !y ...


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6

!mplicit Parameter"se!f#

. Ja)a' t$is, implicitp&lic 'oi! translate(int !x int !y)

x %= !x* '' this(% ) d%+ y %= !y* '' this(y ) dy++

. #ython' sel, e&plicit!e translate(se!f !x !y): se!f(% %= !x se!f(y %= !y

/ E&ercise' 6rite !istance, set,location, and!istance,rom,oriin methods%


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$%ercise &ns'er

point.py

12345

6789

1011121314151617

rom mat$ import

class Point: x = 0 y = 0

!e set,location(sel x y): sel.x = x sel.y = y

!e !istance,rom,oriin(sel): retrn s/rt(sel.x sel.x % sel.y sel.y)

!e !istance(sel ot$er): !x = sel.x - ot$er.x !y = sel.y - ot$er.y retrn s/rt(!x !x % !y !y)


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Calling Methods

. 7 client can call the methods o! an object in to ays'/ 2the )alue o! sel can be an implicit or e&plicit

parameter3

83 object.method(parameters) or

93 Class.method(object parameters)

. E&ample'p = Point(3 -)

p(trans!ate(1 5)Point(trans!ate( p 1 5)


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Constr(ctors

!e ,,init,, (sel, parameter$ ...$ parameter): statements

/ a constructor is a special method ith the name ,,init,,

/ E&ample'class Point:

!e ,,init,,(sel x y):

sel.x = x

sel.y = y

...

. :o ould e make it possible to construct aPoint() ith no parameters to *et 2-, -3


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to-trin. and ,,str,,

!e ,,str,, (sel): retrn string

/ e


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Complete Point Class

point.py

123456

7891011121314

15161718192021

rom mat$ import

class Point: !e ,,init,,(sel x y): sel.x = x sel.y = y

!e !istance,rom,oriin(sel): retrn s/rt(sel.x sel.x % sel.y sel.y)

!e !istance(sel ot$er): !x = sel.x - ot$er.x !y = sel.y - ot$er.y retrn s/rt(!x !x % !y !y)

!e translate(sel !x !y): sel.x %= !x sel.y %= !y

!e ,,str,,(sel): retrn "(" % str(sel.x) % " " % str(sel.y) % ")"


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Operator O)erloading

. operator o)erloading' =ou can de0ne !unctions sothat #ython5s built+in operators can be used ithyour class%

. See also' http'((docs%python%or*(re!(customi>ation%htmlOperator Class Method

- ,,ne,,(sel ot$er)

% ,,pos,,(sel ot$er)

,,ml,,(sel ot$er)

,,tre!i',,(sel ot$er)Unary Operators

- ,,ne,,(sel)

% ,,pos,,(sel)

Operator Class Method

== ,,e/,,(sel ot$er)

= ,,ne,,(sel ot$er)

4 ,,lt,,(sel ot$er)

,,t,,(sel ot$er)

4= ,,le,,(sel ot$er)

= ,,e,,(sel ot$er)

http://docs.python.org/ref/customization.htmlhttp://docs.python.org/ref/customization.htmlhttp://docs.python.org/ref/customization.html


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13

$%ercise

. E&ercise' *rite a /raction class to represent rationalnumbers like 8(9 and +(?%

. @ractions should alays be stored in reduced !orm; !ore&ample, store A(89 as 8( and B(+ as +9(%

/ :int' 7 DC4 2*reatest common di)isor3 !unction mayhelp%

. 4e0ne a!! and mltiply methods that accept another

6raction as a parameter and modi!y the e&istin*6raction by addin*(multiplyin* it by that parameter%

. 4e0ne %, , ==, and 4 operators%


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+enerating $%ceptions

raise $%ceptionype("message")

/ use!ul hen the client uses your object improperly

/ types' rit$metic8rror, ssertion8rror, 9n!ex8rror,ame8rror, yntax8rror, Type8rror, ;ale8rror

/ E&ample'

class


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!nheritance

class name(s(perclass): statements

/ E&ample'class Point3D(Point): # Point3 e%tends Point

> = 0 ...

. #ython also supports multiple inheritance

class name(s(perclass$ ...$ s(perclass): statements

(if > 1 superclass has the same eld/method, conicts are resolved in left-to-right order)

C lli - l


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Calling -(perclassMethods

. methods' class.method(objectparameters)

. constructors' class.,,init,,(parameters)

class Point3D(Point):

> = 0

!e ,,init,,(sel x y >):

Point(,,init,,se!f$ %$ y sel.> = >

!e translate(sel !x !y !>):

Point(trans!atese!f$ d%$ dy sel.> %= !>

08-classes-objects.ppt

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