Object-Oriented Programming in Python Goldwasser and Letscher Chapter 6 Defining Our Own Classes Terry Scott University of Northern Colorado 2007 Prentice Hall

2 Introduction: Chapter 6 Topics Simple point class. Robust point class. Television class. Fraction class. advanced lessons.

3 Predefined versus User Defined Classes Why do classes? It supports reuse of code. Some predefined classes are int, float, list, tuple, boolean, and file. By being able to create our own classes we can configure Python to handle new data types.

4 Point Class class Point: __init__ called the constructor. Executed when a new object is created from a class. self lets Python know that a method or data is a member of the class. –For a data member it should be; self._x where _x is the class data attribute. Without the self a variable is only a local variable inside a class method. –For a method the self is the first parameter in the parameter list. __init__(self,...)

5 Indentation Pattern for a Class

6 Connection Between a Method (setX) for the object corner and setX definition in the class.

7 Two Perspectives Left: perspective from outside the Point class. Right: perspective from inside the Point class.

8 Accessors and Mutators Accessors and mutators let users of the class access data member and change data member values. getX(self) can return the X data member. setX(self, val) will change the X data member to be val.

9 Creating a point class class Point: def __init__(self): self._x = 0 self._y = 0 def getX(self, val): return self._x

10 Point Class (Continued) def setX(self, val): self._x = val def setY(self.val): self._y = val def setY(self, val): self._y = val

11 Using the Point Class #create a new object corner of type Point from SimplePoint import Point corner = Point() corner.setX(8) #8 is value of _x in object corner corner.setY(6) #6 is value of _y in object corner

12 Improved Point class #if no values are specified for x and y then #the values are set to 0. def __init__(self, initX = 0, initY = 0) self._x = initX self._y = initY #Scales the point by a value factor. def scale(self, factor): self._x *= factor self._y *= factor

13 Improved Point Class (continued) def distance(self, other): dx = self._x - other._x dy = self._y – other._y return sqrt(dx*dx + dy*dy) #using the distance method point1 = Point(5,20) point2 = Point(45,60) apartAmt = point1.distance(point2)

14 Improved Point Class (continued) #normalize point – make its distance to the #origin 1 def normalize(self): mag = self.distance(Point()) #Point() creates new point at origin if mag > 0: #don't scale if point is at origin self.scale(1/mag)

15 Improved Point Class (continued) #allow print to be able to print a point object. def __str__(self): return ' ' #using __str__ method new = Point(3, 5) print new #output

16 Improved Point Class (continued) Can not use to initialize an object. point = #this is an error Can overload most operators so that they have a new meaning when used with new objects. An example is + operator when used with int and float does addition. When used with str it does concatenation (sticks the two strings together).

17 Improved Point Class (continued) #overloading operators: + overloading def __add__(other): return Point(self._x +other._x, self._y+other._y #using the __add__ method new = Point(3, 5) old = Point(4, 7) total = new + old print total #output

18 Polymorphism Operator may do a different operation depending on the type that is passed into the operator. Multiplication operator: int or float multiply each component by the value, point do a dot product. isinstance(variable, Type) returns True if variable is of type Type.

19 Polymorphism #if val is an int or float it does the if code #if a Point it does the elif code. def __mul__(self, val): if isinstance(val, (int, float)): #performs regular multiplication operation. return Point(self._x*val, self._y*val) elif isinstance(val, Point): #performs dot product operation. return self._x*val._x + self._y*val._y

20 Television Class Create a user class to emulate the way television controls work. General principles –On-off and mute are both toggle switches –All controls only work when the TV is on. –Volume control goes from1 to 10 inclusive. –Channels range from 2 – 99 inclusive. It wraps around. –Can change channel by entering a channel number.

21 Television Class #initializes television object class Television: def __init__(self): self._powerOn = False self.muted = False self._volume = 5 self._channel = 2 self._prevChan = 2

22 Television Class Diagram

23 Television Class (continued) #Clicking flips if on then off and off then on def togglePower(self): self._powerOn = not self._powerOn #Clicking flips between muted and unmuted. def toggleMute(self): if self._powerOn: self._muted = not self._muted

24 Television Class (continued) #volume can range from 1 upto including 10 def volumeUp(self): if self._powerOn: if self._volume < 10: self._volume += 1 self._muted = False return self_volume #volume is #displayed on tv when it is changed.

25 Television Class (continued) #channel increases by one and wraps back to 2 def channelUp(self): if self._powerOn: self._prevChan = self._channel if self._channel == 99: self._channel = 2 else: self._channel += 1 return self._channel

26 Television Class (continued) volumeDown is similar to volumeUp just replace test self._volume 1 and replace self._volume += 1 with self._volume -= 1 channelDown is similar to channelUp just replace test self._channel == 99 with self._channel == 2 and replace self._channel += 1 with self._channel -= 1.

27 Television Class (continued) #Channel is set to number. def setChannel(self, number): if self._powerOn: if 2 <= number <= 99: self._prevChan = self._channel self._channel = number return self._channel

28 Trace of setChannel(7) Tuned to Channel 5 and Was Previously on Channel 2

29 Television Class (continued) #Flip to previous channel. def jumpPrevChannel(self): if self._powerOn: incoming = self._channel self._channel = self._prevChan self._prevChan = incoming return self._channel

30 Flawed Swap of Channels #Left picture is starting situation self._channel = self._prevChan self._prevChan = self._channel #both variables end up at 5

31 One Way to Do This in Python self._channel, self._prevChan=self.prevChan,self._channel

32 Television Class (continued) # previous code #using swap from previous slide incoming = self._channel self._channel = self._prevChan self._prevChan = incoming # in Python can do the following: can assign #simultaneously so no need to set to a #tempory value. self._channel, self._prevChan=self.prevChan,self._channel

33 Fraction Class Fraction consists of two numbers: a numerator and a denominator. Must be in lowest terms. Denominator must be non-negative. Denominator must be non-zero. Method types: –constructor. –arithmetic operators. –comparison operators. –type conversion operators.

34 Fraction Class Code def __init__(self, numer=0, denom= 0): if denom == 0: self._num = 0 self._den = 0 else: factor = gcd(abs(numer), abs(denom)) if denom < 0: factor = -factor self._num = numer // factor self._den = denom // factor

35 Fraction Class Code (continued) #overloads + operator for fraction objects. def __add__(self, other): return Fraction(self._num*other._den + self._den*other._num, self._den*other._den) #overload < operator for fraction objects. def __lt__(self, other): return self._num*other._den<self._den*other._num

36 Fraction Class Code (continued) # Overloads == operator for fraction objects def __eq__(self, other): return self._num == other._num and self._den == other._den # Overloads float operator for fraction object def __float__(self): return float(self._num) / self._den

37 Fraction Class Code (continued) # overloads int function def __init__(self): return int(float(self)) # overload str function def __str__(self): if self._den == 0: return 'undefined' elif self._den == 1 return str(self._num) else: return str(self._num) + '/' + str(self._den)

38 Complete Fraction Class from gcd import gcd class Fraction: def __init__(self,...) #include constructor code def __add__(self,...) #include + operator code def __sub__(self,...): #similar to __add__ code just - instead

39 Complete Fraction Class (continued) def __mul__(self,other): return Fraction(self._num*other._num, self._den * other._den def __mul__(self,other): return Fraction(self._num*other._den, self._den * other._num #include other comparison operators #include float, int, and str conversion operators.

40 Advanced Lessons #Class-Level Attributes # don't hardwire values such as 0 and 10 # volume and 2 and 99 for channels class Television: #class level attributes will be shared by all _minVolume = 0 _maxVolume = 10 _minChannel = 2 _maxChannel = 99 #useTelevision. _minVolume etc instead of hardwired #values that were used before.

41 Advanced Lessons (continued) # Methods that call other methods # By using previously defined setChannel method can # simplify the code. def channelUp(self): if self._powerOn: if self._channel == Television._maxChannel: goto = Television._minChannel else: goto = self._channel + 1 return self.setChannel(goto)

42 Advanced Lessons (continued) # By separating the wrap-around character- # istics from the actual channel changing the # code is simplified. # Revised jumpPrevChannel using a # previously defined method. def jumpPrevChannel(self): return self.setChannel(self._prevChan)