Exceptions and Making our own classes

1. Exceptions and Making our own classes Many slides from this class are from those provided with the text, created by Terry Scott, University of Northern Colorado. However, I will annotate most of them and add others as needed.

2. Exceptions: How to Deal with Error Situations number = 0 while not 1 <= number <= 10: try: number= int(raw_input('Enter number from 1 to 10: ')) if not 1 <= number <= 10: print 'Your number must be from 1 to 10:' exceptValueError: print 'That is not a valid integer.' Here: recognize an error condition and deal with it If the named error occurs, the “except” clause is executed and the loop is terminated. book slide

3. Exceptions (continued) • What if a negative is entered for square root? • Can raise an exception when something unusual occurs. defsqrE(number): if number < 0: raiseValueError('number must be positive') #do square root code as before Note: ValueError is an existing, defined error class book slide

4. Exceptions (continued) #What if value entered is not a number? defsqrtF(number): if notisinstance(number, (int, float)): raise TypeError('number must be numeric') if number < 0: raise ValueError('number must be positive') #do square root code as before book slide

5. How Much Type Checking is Enough? • A function with little type checking of its parameters may be difficult to diagnose errors. • A function with much type checking of its parameters will be harder to write and consume more time executing. • If the function is one that only you will use you may need less type checking. • A function written for others should have more type checking. book slide

6. Spot check on Exceptions • In pairs, write python code to do the following: • Accept input from the keyboard • Prompt with instructions to enter a number, and to enter some character you choose to end • Verify that the value read is numeric • Calculate the minimum, maximum and average of the values read • Terminate reading when a non numeric character is entered • Print out the values calculated

7. Online class • This is an online class. My travel is not allowing me to do voice-overs of the slides, so I will have to put all my comments into the slides themselves. • Also, because the travel includes all day meetings, including dinner sessions, I don’t have the time to construct very many original slides. I have read the text chapter, will do some slides to provide an overview of the chapter, and rely on the slides provided with the book for most of the content. • I will try to test all the code, because there have been errors in slides I used previously.

8. Overview • This class is about object-oriented programming, in Python. • We focus on the Python parts a lot • This chapter is very much about the object orientation. • At the beginning, we described classes and objects. • Along the way, we have seen built-in functions, and have learned to create our own functions • We have imported modules, such as math • Last week, in exploring the nltk, we learned to work with a large collection of custom materials designed for natural language processing. • Now, we will see how to extend Python to meet our specific needs, by defining our own classes.

9. Mechanics • I will embed spot checks and other questions in the slides. • You may expect that next week’s quiz will include some of those questions and exercises, or things that are very closely patterned on these. Skip them at your peril!

10. Recall from first week • A class is a definition of a category of objects and the methods that operate on those objects • A specific instance of a class is called an object • A class may be related to other classes, and inherit properties from the other class or extend the properties of another class. • A class student is a subclass of person • A student has all the characteristics of a person, but has other characteristics as well (perhaps major, gpa, etc.)

11. Backward references • We will use examples that were introduced earlier. Look back at the previous slides and look at the book to find the details that were provided then. • Examples: • greatest common divisor • television class

12. This class session • By the end of this session, you should be able to define a class, store it in a file, import it and use it in another program. • self as a parameter • local variables • parameters and arguments • special operators • polymorphism • All the ideas will be introduced briefly, then explained by way of examples

13. Defining a class • Similar to defining a function, which we have done. • We saw earlier that a class consists of attributes for the data and methods for operating on the data. • These get translated into Python code as • references to self • functions defined on self, other parameters • class introduces a class definition • classes can reference each other

14. Point Class This is what allows us to instantiate an instance of the class – an object. • 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, . . .)

15. Indentation Pattern for a Class So, this one class includes three methods (function definitions) Notice the indentation: Each function definition is indented and the body of each function is indented within the function.

16. Creating an instance (an object) from a class • __init__(self) defines the structure of the objects of this class. There are two components to objects of this class: _x and _y (yes, the _ is a significant part of the name.) • When a program uses the Point class, the __init__ function establishes its parts and assigns each of the parts the value 0 in this example. • Creating a Point object: a = Point() class Point: def __init__(self): self._x = 0 self._y = 0 a is now a Point object. It consists of two numbers, which we might think of as the x and y components of the Point

17. The setX, setY methods • We can set the value of the Point a: • a = Point() • a.setX(5) • a.setY(2) def setX(self, val): self._x = val _x and _y make these parameters invisible to the caller. def setY(self, val): self._y= val a is now the point <5,2> (using < > to avoid confusion with () and [] which already have meaning.)

18. Referencing the components of the Point • Since _x and _y are hidden, the program cannot reference • a.x or a.y • Instead, • a.getX() • a.getY() Next example uses corner instead of a as the name of the Point.

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

20. Two Perspectives • Left part of each box: perspective from outside the Point class. • Right part of each box: perspective from inside the Point class. Argument used to reference the object’s methods from the program Parameter used to define the object

21. Accessors and Mutators • Accessorsand mutatorslet users of the class access data members and change data member values. • getX(self) can return the X data member. • This is an accessor • setX(self, val) will change the X data member to be val. • This is a mutator

22. Summary: Creating a point class class Point: def __init__(self): self._x = 0 self._y = 0 def getX(self, val): returnself._x

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

24. 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 This assumes that we have created a file with the SimplePoint class definition.

25. #Example SimplePoint code from Chapter 6 of text class Point: def __init__(self): self._x = 0 self._y = 0 def __str__(self): return '<'+str(self._x)+','+str(self._y)+'>’ def getX(self): return self._x def setX(self, val): self._x = val def getY(self): return self._y def setY(self, val): self._y = val This code stored in file named SimplePoint.py from SimplePoint import Point a=Point() a.setX(5) a.setY(2) b = Point() b.setX(-8) b.setY(-3) print "a is ", a, " b is ",b This code stored in file named class-import-test.py

26. #Example SimplePoint code from Chapter 6 of text class Point: def __init__(self): self._x = 0 self._y = 0 def __str__(self): return '<'+str(self._x)+','+str(self._y)+'>’ def getX(self): return self._x def setX(self, val): self._x = val def getY(self): return self._y def setY(self, val): self._y = val Class name match This code stored in file named SimplePoint.py File name match from SimplePoint import Point a=Point() a.setX(5) a.setY(2) b = Point() b.setX(-8) b.setY(-3) print "a is ", a, " b is ",b This code stored in file named class-import-test.py

27. Check Point • Create a class • Make a class called Echo • It has one parameter, a string • It uses __init__ to instantiate an object and __str__ to cast the object as a string for printing and return the string duplicated, with a space between the copies. • Create a calling program to obtain a string from the user and call Echo to print it out twice vu50390:ch06 lcassel$ vu50390:ch06 lcassel$ python useecho.py Enter your string Greetings!! Greetings!! Greetings!! vu50390:ch06 lcassel$ Here is a sample run

28. 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

29. 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) Note – this requires import math or from math import sqrt

30. 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)

31. Improved Point Class (continued) #allow print to be able to print a point object. def __str__(self): return '<'+str(self._x)+', '+str(self._y)+ '>' #using __str__ method new = Point(3, 5) print new #output <3, 5>

32. Improved Point Class (continued) • Can not use <3,5> to initialize an object. point = <3,5> #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).

33. Improved Point Class (continued) #overloading operators: + overloading def __add__(other): returnPoint(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 <7, 12>

34. 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.

35. 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

36. Spot Check • Do exercise 6.4 in the text. Use the Discussion Board in Blackboard to discuss the question with others in the class.

37. 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.

38. 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

39. Television Class Diagram

40. Television Class (continued) #Clicking flips if on then off and off then on deftogglePower(self): self._powerOn = notself._powerOn #Clicking flips between muted and unmuted. deftoggleMute(self): ifself._powerOn: self._muted = not self._muted

41. 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.

42. 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

43. Television Class (continued) • volumeDown is similar to volumeUp just replace test self._volume < 10 with 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.

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

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

46. 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

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

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

49. 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

50. Check Point • Practice Problem 6.5 in the text. • Again, use the Blackboard discussion board to work with classmates to solve this and to explore any questions that arise.