1. More Control and Expanded I/O options

2. Part 1: Defining classes 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

3. 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.)

4. 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

5. 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

6. 6 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,...) This is what allows us to instantiate an instance of the class – an object.

7. 7 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.

8. 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

9. 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 def setY(self, val): self._y = val _x and _y make these parameters invisible to the caller. a is now the point (using to avoid confusion with () and [] which already have meaning.)

10. 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.

11. 11 Connection Between a Method (setX) for the object corner and setX definition in the class.

12. 12 Two Perspectives Left part of each box: perspective from outside the Point class. Right part of each box: perspective from inside the Point class. Parameter used to define the object Argument used to reference the object’s methods from the program

13. 13 Accessors and Mutators Accessors and mutators let 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

14. 14 Summary: Creating a point class class Point: def __init__(self): self._x = 0 self._y = 0 def getX(self, val): return self._x def getY(self, val): return self._y Point has two components, x and y, initially set to 0 Point has methods that allows its x and y components to be referenced

15. 15 Point Class (Continued) def setX(self, val): self._x = val def setY(self.val): self._y = val Point has methods that allow its x and y components to be modified.

16. 16 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.

17. #Example SimplePoint code from Chapter 6 of text class Point: def __init__(self): self._x = 0 self._y = 0 def __str__(self): return ' ’ 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

18. #Example SimplePoint code from Chapter 6 of text class Point: def __init__(self): self._x = 0 self._y = 0 def __str__(self): return ' ’ 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 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 Class name match This code stored in file named SimplePoint.py File name match

19. class Echo: def __init__(self, anyStr): self.text = anyStr def __str__(self): return str(self.text) + " " + str(self.text)

20. 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!! vu50390:ch06 lcassel$ Here is a sample run

21. 21 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

22. 22 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

23. 23 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)

24. 24 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

25. 25 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).

26. 26 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

27. 27 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.

28. 28 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

29. Spot Check Do exercise 6.4 in the text. – Enter the code shown. – Execute each of the examples – Explain the results

30. Other examples Look at the descriptions of the Television and Fraction classes in the text for further examples of class definition.

31. More control – Catching exceptions

32. 32 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:' except ValueError: 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

33. 33 Exceptions (continued) What if a negative is entered for square root? Can raise an exception when something unusual occurs. def sqrE(number): if number < 0: raise ValueError('number must be positive') #do square root code as before Note: ValueError is an existing, defined error class book slide

34. 34 Exceptions (continued) #What if value entered is not a number? def sqrtF(number): if not isinstance(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

35. 35 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

36. 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 36

37. Formatting and File access

38. Building on basics We had – Input from the keyboard nameIn = raw_input(“What is your name?”) – and output to the console print “Hello”, nameIn Additions: – default for value not input: nameIn = raw_input(“What is your name?”) if not nameIn: nameIn = “Anonymous” No input provided

39. More additions Printing a simple list of strings includes a space between each pair. – Unwanted space between team name and : – to fix this use concatenation of strings (+ operator). Must explicitly convert numbers to strings. Gain full control of the spacing >>> team = "Wildcats" >>> rank = 5 >>> print team, ": ranked", rank, "this week." Wildcats : ranked 5 this week. >>> >>> print team+": ranked " + str(rank) +" this week." Wildcats: ranked 5 this week.

40. Formatting Strings Further control of how individual fields of output will be presented. – % is used to indicate a formatting code and also a tuple of items to be formatted – %s for strings – %d for integers (d for digits?) – %f for floats (numbers with decimal parts) %.3f displays three decimal places

41. 41 Formatted Strings (continued) Can write previous statement using formatting strings like this. Format strings are: – %s is for strings – %d is for integers – %f is for floats. %.2f gives two decimal places. >>> print '%s: ranked %d this week.'%(team, rank) Wildcats: ranked 5 this week. Notice quotes around the whole specification of the formatting.

42. Formatting details Further options – %10s -- string with 10 spaces, minimum – %4d -- number with 4 spaces, minimum – -%5.2f -- float with 5 places, of which two are decimal positions >>> print 'Rank %5.2f as a float.'%rank Rank 5.00 as a float. >>> print 'Rank %10.2f as a float.'%rank Rank 5.00 as a float. >>> rank = 100 >>> print "Rank %3.2f with field too small"%rank Rank 100.00 with field too small Note: %n.df makes the total columns for the number =n, of which d are for the decimal places %3.2f means total 3 spaces, one is for the decimal point and two for the decimal digits, none for the whole number. Automatically expanded to fit the actual value.

43. Short spot check Write code to print out – Today is 10 October 2012 – Where the date consists of 3 variables Day is an integer Month is a string Year is an integer – Use the formatting tools we just saw and make the day number always 2 digits, the month name a set length (long enough for any month) and the year four digits.

44. 44 Working with Files Information stored in RAM (main memory) goes away (is volatile) when the computer is shut off. Information stored on disk is non- volatile (does not go away when the computer is turned off). Writing to and reading from a file can help preserve information between different executions of a program.

45. 45 Python File Type creating a new file instance is accomplished in the same way that a new list object is made. fileObj = file(filename)

46. 46 File Operations SyntaxSemantics close()disconnect file from Python file variable and save file. flush()flushes buffer of written characters. read()returns a string with remaining contents of the file. read(size)returns a string with size bytes remaining in file. readline()returns string that contains next line in the file.

47. 47 File Operations (continued) SyntaxSemantics readlines()returns a list of strings of the remaining lines in the file. write(s)writes s to the file. No newlines are added. writelines(seq)writes the lines in seq to the file. for line in f:iterates through the line f, one line at a time.

48. 48 Reading from a File: Counting lines, words, and characters version 1 – corrected typos and added formatting filename = raw_input('What is the filename? ') source = file(filename) text = source.read() # Read entire file as one string numchars = len(text) numwords = len(text.split()) numlines = len(text.split('\n')) print '%10d Lines\n%10d Words\n%10d Characters'%(numlines,numwords,numchars) source.close() What is the filename? citeseertermcount.txt 30002 Lines 156521 Words 920255 Characters Note – this version reads the whole file at once, as a single string

49. 49 Reading from a File: Counting lines, words, and characters version 2 numlines=numwords=numchars=0 line=source.readline() while line: # line length is not zero numchars+=len(line) numwords +=len(line.split()) numlines+=1 # Done with current line. Read the next line=source.readline() print '%10d Lines\n%10d Words\n%10d Characters'%(numlines,numwords,numchars) source.close() Now, we read one line at a time, process it, and read the next. What is the filename? citeseertermcount.txt 30001 Lines 156521 Words 920255 Characters Note different number of lines

50. 50 Reading from a File: Counting lines, words, and characters version 3 filename = raw_input('What is the filename? ') source = file(filename) numlines = numwords = numchars = 0 for line in source: #reads one line at a time until no more. numchars += len(line) numwords += len(line.split()) numlines += 1 print '%10d Lines\n%10d Words\n%10d Characters'%(numlines,numwords,numchars) source.close() 30001 Lines 156521 Words 920255 Characters Note that “for line in source” actually does the read of a line. No explicit readline is used. Note the number of lines

51. Spot check Read a file of your choice – anything you have is fine Find the longest line – Output the line number and its length in characters

52. 52 Writing to a File Creating a new file object that can be written to in Python with a file name of filename. result = file(filename, 'w') If the file with filename already exists then it will be overwritten. Only strings can be written to a file pi = 3.14159 result.write(pi) #this is illegal result.write(str(pi)) #this is legal

53. 53 Writing to a File When is the information actually written to a file? File writing is time expensive so files may not be written immediately. A file can be forced to be written in two ways: – flush(): file written but not closed. – close(): file written and then closed.

54. File Write Danger Note that there is no built-in protection against destroying a file that already exists! If you want to safeguard against accidentally overwriting an existing file, what would you do? – Discuss

55. 55 Trying to Read a File That Doesn't Exist. What if opening file for reading and no file with that name exists? IOError – crashes program. To avoid this use an exception. filename = raw_input('Enter filename: ') try: source = file(filename) except IOError: print 'Sorry, unable to open file', \ filename

56. 56 File Utilities # Prompt for filename until file is successfully opened. def fileReadRobust(): source = None while not source: filename = raw_input('Input filename: ') try: source = file(filename) except IOError: print 'Sorry, unable to open file', filename return source

57. 57 File Utilities (continued) def openFileWriteRobust(defaultName): """Repeatedly prompt user for filename until successfully opening with write access. Return a newly open file object with write access. defaultName a suggested filename. This will be offered within the prompt and used when the return key is pressed without specifying another name. """ writable = None while not writable: # still no successfully opened file prompt = 'What should the output be named [%s]? '% defaultName filename = raw_input(prompt) if not filename: # user gave blank response filename = defaultName # try the suggested default try: writable = file(filename, 'w') except IOError: print 'Sorry. Unable to write to file', filename return writable

58. Spot check Enter the code shown for the robust file open. Add a call to this function. How can you test this code?

59. Testing the File Utilities from FileUtilities import * sourceFile=openFileReadRobust() if sourceFile <> None: print "Successful read of ",sourceFile filenone="anyname" outFile=openFileWriteRobust(filenone) if outFile <> None: print "File ", outFile, " opened for writing" What is the filename? citeseertermcount.txt Successful read of What should the output be named [anyname]? abc.txt File opened for writing

60. Numbering lines in a file # Program: annotate.py # Authors: Michael H. Goldwasser # David Letscher # # This example is discussed in Chapter 8 of the book # Object-Oriented Programming in Python # from FileUtilities import openFileReadRobust, openFileWriteRobust print 'This program annotates a file, by adding' print 'Line numbers to the left of each line.\n' source = openFileReadRobust() annotated = openFileWriteRobust('annotated.txt') # process the file linenum = 1 for line in source: annotated.write('%4d %s' % (linenum, line) ) linenum += 1 source.close() annotated.close() print 'The annotation is complete.'

61. 61 Running the annotation program FileUtilities.pycciteseertermcount.txtreadfile1.py abc.txtfileUtilTest.pyreadfile2.py annotate.pyfileUtilities.pyreadfile3.py annotatedUtilities.txtreadexception.py This program annotates a file, by adding Line numbers to the left of each line. What is the filename? fileUtilities.py What should the output be named [annotated.txt]? annotatedUtilities.txt The annotation is complete. Directory after the program runs:

62. The annotated file 1 # Program: FileUtilities.py 2 # Authors: Michael H. Goldwasser 3 # David Letscher 4 # 5 # This example is discussed in Chapter 8 of the book 6 # Object-Oriented Programming in Python 7 # 8 """A few utility functions for opening files.""" 9 def openFileReadRobust(): 10 """Repeatedly prompt user for filename until successfully opening with read access. 11 12 Return the newly open file object. 13 """ 14 source = None 15 while not source: # still no successfully opened file 16 filename = raw_input('What is the filename? ') 17 try: 18 source = file(filename) 19 except IOError: 20 print 'Sorry. Unable to open file', filename 21 return source 22 23 def openFileWriteRobust(defaultName): 24 """Repeatedly prompt user for filename until successfully opening with write access. 25 26 Return a newly open file object with write access. Rest not shown for space limitations

63. Spot Check 2 Run the annotate program against a file of your choosing and get the line numbers added. – Be careful not to overwrite the original file. – Modify the code to remove the file numbers from the file

64. Tally Read through the case study of constructing a tally sheet class. Compare what you see here to the frequency distribution content that you saw in the NLTK book.

65. NLTK chapter 3 That is written very much as a tutorial and I don’t think I can do much with slides and no narration. Please read through that chapter and do the “Your turn” exercises. Use the Discussion board to comment on what you do and to share observations and ask questions.

66. Assignment In Two weeks: – Do either exercise 8.18 or exercise 8.21 – (Do you prefer to work with numbers or words?) – Be sure to design good test cases for your program. For chapter review (and quiz preparation) be sure you can do exercises 8.7 – 8.9