Review

• Inheritance
• Principles of Inheritance
• An Example of Inheritance
• The Video Class
• The Video Constructor
• Validation Methods for Video
• Static Methods
• Calculating the Collection Number
• Writing he Video Constructor
• Video Accessors
• The __str__ Method

New Material

• The Movie Subclass
• Creating a Subclass
• Creating a Constructor for a Subclass
• Movie Accessor Methods
• The add_actor Method
• A __str__ Method for Movie
• The super Function
• Creating Derived Objects
• The Instructional Class
• The Instructional Constructor
• The get_name Method for
• The __str__ Method for Instructional
• Polymorphism
• Mutator for the Instructional Class
• Accessors for the Instructional Class
• The isinstance Function
• Fairness

Graded Quiz 9

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Solution to Homework 9

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Homework 11

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This is the last homework assignment.

It is due this Sunday at 11:59 PM.

Announcement

The UMB IT Club and Boston Linux User Group will hold a Linux InstallFest on Saturday, May 3rd, from 9 to 5 in the McCormack Conference Room, M03-0721.

If you have a machine on which you would like to install Linux and would like some help in doing this, bring it the InstallFest.

Volunteers from the Boston Linux User Group with be on hand to help with the installation.

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The Boston Linux and Unix User Group counts among its members some of the most knowledgeable Linux and Unix people in the Boston area.

You will find directions to M03-0721 here

Questions

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Review

Inheritance

• Inheritance lets you use one class as the basis for another
• The new class will have all the attributes and methods of of the first

Principles of Inheritance

• For inheritance to work the two classes must have a certain relationship
• An "is a" relationship
• A truck is a vehicle
• So a Truck class can inherit from a Vehicle class
• Truck will have all the attributes and methods of Vehicle ...
• in addition to some new attributes and methods
• Whenever we use one class as the basis of another class ...
• we call the original class the superclass or the base class
• The class which inherits is called the subclass or derived class
• A subclass inherits all the attributes and methods of its superclass

An Example of Inheritance

• I have over 500 DVDs of various types
• Some are movies
• Others are television series such as Mythbusters or Nova
• I also have video lectures that I buy from The Great Courses
• I need to store different information for each type of video
• A Great Courses Video always contains 6 lessons
• Movies always have a director
• But TV series do not

The Video Class

• Let's start out by creating the superclass Video
• Here are the attributes
  • Collection number
  • Name
  • Length
  • Format
• The collection number is a unique number assigned to each video entry ...
• starting with 1
• Length is the running time in minutes
• The format can either be "DVD" or "Blue Ray"
• None of these attributes will change after they have been entered
• So I need no mutators
• I only need accessors ...
• and a constructor

The Video Constructor

• You might think the constructor would look like this

  def __init__(self, collection_no, name, length, format):
      self.__collection_no = collection_no
      self.__name   = name
      self.__length = length
      self.__format = format

• But I need to make sure the values of the parameters are reasonable
• So I need to create some validation methods ...
• which will be called by the constuctor

Validation Methods for Video

• Although there are 4 attributes in the Video class
• The constructor will only require three values
  • Name
  • Length
  • Format
• The value for collection_no will be calculated ...
• which I will do in the next section
• I have to specify the proper values for each parameter
• Here they are
  • Name - a string of at least 3 characters that are letters or digits
  • Length - an integer ranging from 15 to 1000
  • Format - "DVD" or "Blue Ray"
• __check_name has to count the characters in name that are either letters or digits
• We do this in a for loop using the string method isalnum
• If the number of characters is less than what is required ...
• the code will raise a ValueError exception
• The global variable MIN_NAME_LENGTH will hold the minimum number of characters
• Here is the code

  MIN_NAME_LENGTH = 3
  
  class Video:
  ...
  
      def __check_name(name):
          if type(name) is not str:
              raise TypeError("Video constructor expects a string argument for name")
          char_count = 0
          for char in name:
              if char.isalnum():
                  char_count += 1
          if char_count < MIN_NAME_LENGTH:
              raise ValueError("Name must have at least " + str(MIN_NAME_LENGTH) + \
                  " characters that are letters or digits")

• The value for the length attribute must be an integer ...
• between 15 and 1000
• Here is the code

  MIN_LENGTH      = 15
  MAX_LENGTH      = 1000
  
  class Video:
  ...
      def __check_length(length): 
          if type(length) is not int:
              raise TypeError("Video constructor expects a integer argument for length")
          if length < MIN_NAME_LENGTH or length > MAX_LENGTH:
              raise ValueError("Length must be at least " + str(MIN_LENGTH) + 
                  " and no more than " + str(MAX_LENGTH))

• Format can be either "DVD" or "Blue Ray"
• FORMATS is a global variable holding the legal values
• Here is the code

  FORMATS         = ("DVD", "Blue Ray")
  
  class Video:
  ...
      def __check_format(format):
          if type(format) is not str:
              raise TypeError("Video constructor expects a string argument for format")
          if format not in FORMATS:
              raise ValueError("Format must be one of " + str(FORMATS))

Static Methods

• If you look carefully at the headers of the validations methods ...
• you will see something odd
• None of them take self as a parameter
• There is no need for them to use self
• They only work with the value in their single paramater
• They do not access any attributes of the class
• Methods like this are called static methods
• They belong to the class itself
• Not to a specific instance of a class

Calculating the Collection Number

• __collection_no should be calculated automatically
• I need to store the last number used
• This value will initially be set to zero ...
• and will be incremented for each video entry
• The value must remain even after the script ends
• So I will store it as a string in a file on disk
• The filename will be stored in the global variable FILENAME
• __next_collection_no will
  • Read the last number from the file
  • Increment that number
  • Save the new number to the file
  • Return the new number to the calling function
• If the file does not exist, __next_collection_no will
  • Set the next number to 1
  • Write the number to the file
  • Return the number
• Here is the code

  # reads the last collection number used from a file,
  # increments that number and writes it back to the file
  # and returns the incremented number
  def __next_collection_no():
      try:
          file = open(FILENAME, 'r')
          collection_no = int(file.readline())
      except FileNotFoundError:
          collection_no = 1
      else:
      	collection_no += 1
          file.close()
      finally:
          file = open(FILENAME, 'w')
          file.write(str(collection_no))
          file.close()
          return collection_no

Writing he Video Constructor

• The constructor will first call the validation methods
• Then call __next_collection_no() ..
• and set the attributes

  def __init__(self, name, length, format):
      self.__check_name(name)
      self.__check_length(length)
      self.__check_format(format)
      self.__collection_no = self.__next_collection_no()
      self.__name   = name
      self.__length = length
      self.__format = format

Video Accessors

• We need accessors for each of the attributes

  def get_collection_no(self):
      return self.__collection_no
  
  def get_name(self):
      return self.__name
  
  def get_length(self):
      return self.__length
  
  def get_format(self):
      return self.__format

The __str__ Method

• __str__ will return the attribute values

  def __str__(self):
      return str(self.__collection_no) + ": " + self.__name + ", " + \
          str(self.__length) + " minutes, " + self.__format

• You can see the code for the class here

Attendance

New Material

The Movie Subclass

• Let's use Video to define specific subclasses
• Most of the entries in my video collection are movies
• Every movie entry will have the four attributes of the Video class
  • Collection number
  • Name
  • Length
  • Format
• But it will also have the following additional attributes
  • Director
  • Studio
  • Actors
• I must decide what attributes I want set by the constructor
• I certainly want it to set __director and __studio
• But there will be many actors for each movie
• So I will add them later
• __actors will be a set
• Actors will be added to this set by a mutator

Creating a Subclass

• I am going to create a separate module for the Movie class
• It will be a subclass of Video ...
• so I will have to import the parent class

  from video import Video

• video is the module
• And Video is the class contained in that module
• In declaring Movie we need to say it is a subclass of Video
• We use a different format for doing this

  class SUBCLASS_NAME(SUPERCLASS_NAME):

• So we declare Movie as follows

  class Movie(Video):

Creating a Constructor for a Subclass

• We need to give the Movie constructor 5 arguments
  • name
  • length
  • format
  • director
  • studio
• But the Movie constructor will not set the first three
• It will call the Video constructor to do that
• We need to use dot notation to call that constructor
• But what should we put before the dot?
• If you think about it, __init__ does not belong to the object
• It belongs to the class itself
• So we refer to the __init__ method of the Video class like this

  Video.__init__(...)

• Then we need to set the Movie attributes
  • directory
  • studio
  • actors
• The actors will be stored in a set
• And the names will be added after the constructor is called
• So at this point all we need to do is create an empty set
• Here is the code for the Movie constructor

  def __init__(self, name, length, format, director, studio):
      Video.__init__(self, name, length, format)
      self.__director = director
      self.__studio   = studio
      self.__actors   = set()

• Which we test

  >>> from movie import Movie
  >>> m1 = Movie("Forbidden Planet", 98, "DVD", "Fred McLeod Wilcox", "MGM")
  >>> str(m1)
  '1: Forbidden Planet, 98 minutes, DVD'

• Notice that we did not have to create a __str__ method for Movie
• Movie is a subclass of Video
• So it inherits the __str__ method from Video class ...
• along with the accessors

  >>> m1.get_collection_no()
  1
  >>> m1.get_name()
  'Forbidden Planet'
  >>> m1.get_length()
  98
  >>> m1.get_format()
  'DVD'

Movie Accessor Methods

• We need accessors for each of three new Movie attributes

  def get_director(self):
      return self.__director
  
  def get_studio(self):
      return self.__studio
  
  def get_actors(self):
      return self.__actors

• Which we test

  >>> m1.get_director()
  'Fred McLeod Wilcox'
  >>> m1.get_studio()
  'MGM'
  >>> m1.get_actors()
  []
  

The add_actor Method

• The names of actors are added after the object has been created
• So we need a mutator to add the names

  def add_actor(self, name):
          self.__actors.add(name)

• Which we test

  >>> m1.add_actor("Walter Pidgeon")
  >>> m1.add_actor("Anne Francis")
  >>> m1.add_actor("Leslie Nielson")
  >>> m1.add_actor("Warren Stevens")
  >>> m1.get_actors()
  ['Walter Pidgeon', 'Anne Francis', 'Leslie Nielson', 'Warren Stevens']

A __str__ Method for Movie

• Movie has more attributes than Video
• But the __str__ it inherits from Video will not show them
• We could try to create a new __str__ method like this

  def __str__(self):
      return str(self.__collection_no) + ": " + self.__name + ", " + \
          str(self.__length) + " minutes, " + self.__format + \
          " Directed by " + self.__director + ", " + self.__studio

• But this won't work

  >>> str(m1)
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/Users/glenn/workspace-mars/it117/resources_it117/code_it117/example_code_it117/11_chapter_example_code/movie.py", line 26, in __str__
      ' Directed by ' + self.__director + ', ' + self.__studio
  AttributeError: 'Movie' object has no attribute '_Movie__collection_no'

• The first four attributes are contained in Video
• Not in Movie
• We need a special function to access the methods in the superclass

The super Function

• You cannot access the methods in a superclass object directly
• Instead you have to use the built-in function super
• A call to super returns something like a variable ...
• that points to the superclass object
• You use dot notation with this variable to call the superclass's methods
• Like this

  def __str__(self):
      return super().__str__() + " Directed by " + self.__director + ", " + self.__studio

• Which works when we test it

  >>> str(m1)
  '9: Forbidden Planet, 98 minutes, DVD Directed by Fred McLeod Wilcox, MGM'

• You will file the source code for the Movie class here

Creating Derived Objects

• Whenever we create an object of a derived class
• We are actually creating two objects ...
• that behave as one
• An object of the derived class
• And an object of the base class
• So when we create a Movie object
• We are also creating a Video object
• The picture in memory looks like this

• The object variable m1 points to the derived object
• And super connects to the base class object

The Instructional Class

• Let's create another subclass of Video
• I buy videos from The Great Courses
• The videos come in a set for a specific course
• Taught by a professor in the field
• Each individual DVD has 6 lectures ...
• of 30 minutes each
• Each course is contained on several DVDs
• And each DVD will have its own entry
• Here is the list of attributes for this class
  • __course_name
  • __company
  • __disc_number
  • __instructor
  • __lectures
• The first 4 will be set by the constructor
• __lectures will be a list ...
• containing the names of each of the lectures
• The entries in this list will be set with a mutator
• Why did I chose a list instead of a set ...
• as I did with the Movie class ...
• The lectures need to be viewed in order
• The elements of a list are ordered ...
• but not those in a set

The Instructional Constructor

• We face the same issues in Instructional that we faced in Movie
• But there is a new problem
• The __name field will be empty
• I'll explain why a little bit later
• The constructor for our new class must call the Video constructor
• So we have to supply a value for name
• I will use the nonsense string "XXX" as the value
• All calls to the Video constructor will have the same value for __name
• So I will not need a name parameter for the Instructional constructor
• The parameters will also be in a different order
• Which makes more sense for this type of video entry
• Here is the code

  def __init__(self, course_name, company, disc_number, instructor, length, format):
      Video.__init__(self, "XXX", length, format)
      self.__course_name = course_name
      self.__company     = company
      self.__disc_number = disc_number
      self.__instructor  = instructor
      self.__lectures = []

The get_name Method for Instructional

• In the Video and Movie class the name attribute holds the name of the video
• This value is set in the Video constructor
• But I want the name for an Instructional object to be different
• I want it to have the following format

  COURSE_NAME: Disc DISC_NO

• In other words, I want the name value to be calculated from other values ...
• not stored in the name attribute
• This means I cannot use the version of get_name inherited from the Video class
• This class needs its own version of get_name

  def get_name(self):
      return self.__course_name + ': Disc ' + str(self.__disc_number)

• Which works when I test it

  >>> i1.get_name()
  'Understanding the Universe: Disc 1'

The __str__ Method for Instructional

• In the Movie class __str__ called the __str__ method of Video ...
• and then appended the Movie attributes
• But if we call

  super().__str__()

  as we did in Movie
• We will get "XXX" in the output
• This is not what I want
• I want the string representation of my Instructional videos to have a specific format

  COLLECTION_NO: COURSE_NAME: Disc DISC_NO, INSTRUCTOR

• This means the only information I need from the Video superclass ...
• is the collection_no
• So __str__ will call get_collection_no from the Video class ...
• along with it's own version of get_name ...
• as well as value of __instructor
• Here is the code

  def __str__(self):
      return str(super().get_collection_no()) + ': ' + self.get_name() + ', ' + self.__instructor

• Notice that I did not use all of the attributes from the superclass
• __str__ doesn't need to show all the attributes
• It just needs to create a reasonable string representing the object
• Here is the test

  >>> from instructional import Instructional
  >>> i1 = Instructional("Understanding the Universe", "Great Courses", 1, "Alex Filippenko", 180, "DVD")
  >>> str(i1)
  '1: Understanding the Universe: Disc 1, Alex Filippenko'

Polymorphism

• The methods we declare in a superclass ...
• will be available in all subclasses
• When a subclass has a method with the same name as the superclass ...
• the interpreter will use the subclass method
• The subclass method is said to override the superclass method
• This means that we can call any of the methods in the superclass ...
• on any instance of the subclass
• Even though the definition of the method can be different in each subclass
• So we can create a list of subclass objects ...
• calling the same method on each one ...
• but getting very different results

  >>> from movie import Movie
  >>> m1 = Movie("Forbidden Planet", 98, "DVD", "Fred McLeod Wilcox", "MGM")
  >>> from instructional import Instructional
  >>> i1 = Instructional("Understanding the Universe", "Great Courses", 1, "Alex Filippenko", 180, "DVD")
  >>> videos = []
  >>> videos.append(m1)
  >>> videos.append(i1)
  >>> for video in videos:
  ...     print(video)
  ...     print(video.get_name())
  ... 
  2: Forbidden Planet, 98 minutes, DVD Directed by Fred McLeod Wilcox, MGM
  Forbidden Planet
  1: Understanding the Universe: Disc 1, Instructor Alex Filippenko
  Understanding the Universe: Disc 1
  

• This is an example of a feature of inheritance called polymorphism

Mutator for the Instructional Class

• __lectures is not set by the constructor
• So we need a mutator method for it
• It stores the title of each lecture in the video
• It works just like add_actor in Movie

  def add_lecture(self, lecture):
      self.__lectures.append(lecture)

Accessors for the Instructional Class

• Here are the accessors for the Instructional class

  def get_name(self):
      return str(super().get_collection_no()) + ": " + self.__course_name + ": Disc " + \
          str(self.__disc_number)
  
  def get_course_name(self):
      return self.__course_name
  
  def get_company(self):
      return self.__company 
  
  def get_disc_number(self):
      return self.__disc_number
  
  def get_instructor(self):
      return self.__instructor
  
  def get_lectures(self):
      return self.__lectures

• When I test them, they work as follows

  >>> i1.add_lecture("A Grand Tour of the Cosmos")
  >>> i1.add_lecture("The Rainbow Connection")
  >>> i1.add_lecture("Sunrise, Sunset")
  >>> i1.add_lecture("Bright Objects in the Night Sky")
  >>> i1.add_lecture("Fainter Phenomena in the Night Sky")
  >>> i1.add_lecture("Our Sky Through Binoculars and Telescopes")
  >>> i1.get_course_name()
  'Understanding the Universe'
  >>> i1.get_company()
  'Great Courses'
  >>> i1.get_disc_number()
  1
  >>> i1.get_instructor()
  'Alex Filippenko'
  >>> i1.get_lectures()
  ['A Grand Tour of the Cosmos', 'The Rainbow Connection', 'Sunrise, Sunset', 'Bright Objects in the Night Sky', 'Fainter Phenomena in the Night Sky', 'Our Sky Through Binoculars and Telescopes'
  

• You will find the source code for the Instructional class here

The isinstance Function

• Inheritance is a useful ...
• because it lets us reuse code
• Reusing code is good practice
• The more code we write ...
• the greater the chance of an error
• But inheritance is not without its drawbacks
• What if you have an object that you know is a subclass of Video
• But you don't know which subclass?
• Each subclass has its own unique methods
• If you call a method on an object that does not implement the method ...
• the interpreter will raise an exception
• What can you do?
• You can use the isinstance built-in function
• The general format for using this function is

  isinstance(OBJECT_VARIABLE, CLASS_NAME)

• isinstance is a boolean function that returns True or False

  >>> isinstance(i1, Movie)
  False
  >>> isinstance(i1, Instructional)
  True

• isinstance will also return True ...
• when you use the class name of the superclass

  >>> isinstance(i1, Video)
  True

Class Exercise

Class Quiz