Python 3 Essential Training
Illustration by John Hersey

Python 3 Essential Training

with Bill Weinman

Video: Greater reusability with inheritance and polymorphism

The object model in Python is really very powerful and well-designed. And Python does support inheritance and polymorphism. So let's take a look at an example of how we do inheritance and polymorphism in Python. First, we'll make a working copy of, and we'll open our working copy, and here we have a number of classes, and you'll see we have these animals: Dog, Person and Duck. And each of these inherits AnimalActions.
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  1. 5m 14s
    1. Welcome
      1m 32s
    2. Understanding prerequisites for Python
      2m 4s
    3. Using the exercise files
      1m 38s
  2. 33m 29s
    1. Getting started with "Hello World"
      4m 43s
    2. Selecting code with conditionals
      4m 45s
    3. Repeating code with a loop
      4m 13s
    4. Reusing code with a function
      2m 43s
    5. Creating sequences with generator functions
      2m 46s
    6. Reusing code and data with a class
      4m 39s
    7. Greater reusability with inheritance and polymorphism
      7m 17s
    8. Handling errors with exceptions
      2m 23s
  3. 22m 32s
    1. Installing Python 3 and Eclipse for Windows
      11m 24s
    2. Installing Python 3 and Eclipse for Mac
      11m 8s
  4. 28m 0s
    1. Creating a main script
      3m 27s
    2. Understanding whitespace in Python
      4m 8s
    3. Commenting code
      3m 28s
    4. Assigning values
      3m 37s
    5. Selecting code and values with conditionals
      4m 46s
    6. Creating and using functions
      3m 54s
    7. Creating and using objects
      4m 40s
  5. 31m 23s
    1. Understanding variables and objects in Python
      2m 46s
    2. Distinguishing mutable and immutable objects
      2m 41s
    3. Using numbers
      3m 34s
    4. Using strings
      6m 38s
    5. Aggregating values with lists and tuples
      4m 55s
    6. Creating associative lists with dictionaries
      4m 24s
    7. Finding the type and identity of a variable
      4m 45s
    8. Specifying logical values with True and False
      1m 40s
  6. 9m 42s
    1. Selecting code with if and else conditional statements
      2m 22s
    2. Setting multiple choices with elif
      2m 14s
    3. Understanding other strategies for multiple choices
      2m 38s
    4. Using the conditional expression
      2m 28s
  7. 11m 26s
    1. Creating loops with while
      1m 27s
    2. Iterating with for
      3m 54s
    3. Enumerating iterators
      3m 22s
    4. Controlling loop flow with break, continue, and else
      2m 43s
  8. 23m 28s
    1. Performing simple arithmetic
      2m 14s
    2. Operating on bitwise values
      3m 30s
    3. Comparing values
      3m 32s
    4. Operating on Boolean values
      2m 59s
    5. Operating on parts of a container with the slice operator
      6m 52s
    6. Understanding operator precedence
      4m 21s
  9. 11m 34s
    1. Using the re module
      1m 4s
    2. Searching with regular expressions
      3m 12s
    3. Replacing with regular expressions
      3m 29s
    4. Reusing regular expressions with re.compile
      3m 49s
  10. 9m 10s
    1. Learning how exceptions work
      1m 18s
    2. Handling exceptions
      4m 15s
    3. Raising exceptions
      3m 37s
  11. 23m 1s
    1. Defining functions
      6m 23s
    2. Using lists of arguments
      2m 26s
    3. Using named function arguments
      4m 32s
    4. Returning values from functions
      1m 55s
    5. Creating a sequence with a generator function
      7m 45s
  12. 47m 29s
    1. Understanding classes and objects
      5m 12s
    2. Using methods
      6m 12s
    3. Using object data
      10m 4s
    4. Understanding inheritance
      5m 11s
    5. Applying polymorphism to classes
      7m 13s
    6. Using generators
      9m 48s
    7. Using decorators
      3m 49s
  13. 18m 54s
    1. Understanding strings as objects
      3m 25s
    2. Working with common string methods
      5m 24s
    3. Formatting strings with str.format
      5m 31s
    4. Splitting and joining strings
      2m 49s
    5. Finding and using standard string methods
      1m 45s
  14. 25m 27s
    1. Creating sequences with tuples and lists
      4m 6s
    2. Operating on sequences with built-in methods
      5m 50s
    3. Organizing data with dictionaries
      4m 56s
    4. Operating on character data with bytes and byte arrays
      10m 35s
  15. 11m 46s
    1. Opening files
      2m 4s
    2. Reading and writing text files
      4m 33s
    3. Reading and writing binary files
      5m 9s
  16. 21m 27s
    1. Creating a database with SQLite 3
      6m 56s
    2. Creating, retrieving, updating, and deleting records
      7m 31s
    3. Creating a database object
      7m 0s
  17. 18m 27s
    1. Using standard library modules
      8m 0s
    2. Finding third-party modules
      5m 47s
    3. Creating a module
      4m 40s
  18. 23m 11s
    1. Dealing with syntax errors
      8m 19s
    2. Dealing with runtime errors
      4m 0s
    3. Dealing with logical errors
      4m 22s
    4. Using unit tests
      6m 30s
  19. 19m 56s
    1. Normalizing a database interface
      6m 39s
    2. Deconstructing a database application
      8m 9s
    3. Displaying random entries from a database
      5m 8s
  20. 29s
    1. Goodbye

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Watch the Online Video Course Python 3 Essential Training
6h 36m Beginner Jul 29, 2010

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Due to its power, simplicity, and complete object model, Python has become the scripting language of choice for many large organizations, including Google, Yahoo, and IBM. In Python 3 Essential Training, Bill Weinman demonstrates how to use Python 3 to create well-designed scripts and maintain existing projects. This course covers the basics of the language syntax and usage, as well as advanced features such as objects, generators, and exceptions. Example projects include a normalized database interface and a complete working CRUD application. Exercise files accompany the course.

Topics include:
  • A Python 3 quick start for experienced developers
  • Creating functions and objects
  • Using Python’s built-in objects and classes
  • Repeating code with loops and iterators
  • Understanding and using conditional expressions
  • Creating sequences with generators
  • Reusing code with objects and libraries
  • Handling errors with exceptions
Developer Web
Bill Weinman

Greater reusability with inheritance and polymorphism

The object model in Python is really very powerful and well-designed. And Python does support inheritance and polymorphism. So let's take a look at an example of how we do inheritance and polymorphism in Python. First, we'll make a working copy of, and we'll open our working copy, and here we have a number of classes, and you'll see we have these animals: Dog, Person and Duck. And each of these inherits AnimalActions.

So AnimalActions has the code in it. It has these four methods: quack, feathers, bark and fur. And you will notice that each of these methods is defined on one line. So because each of these methods has just one line of code, you can do it all on one line. You have the def keyword, and then you have the name of the method and then the code. In this case, it's return self.strings 'quack'. This notation here is because strings is a dictionary, and we'll get into that later on in the course when we talk about data types. But these strings, self.strings, are actually defined in the classes that inherit AnimalActions, and so this is an example of inheritance.

This is also an example of data abstraction, because each of these classes has different data. So here we have the duck, and it has these four strings in it: quack, feathers, bark and fur. And the duck says quack. The duck has gray and white feathers. The duck cannot bark. The duck has no fur. And then we have a Person, which also inherits AnimalActions, and the Person has quack, feathers, bark and fur. But for quack, instead of saying quack, the person imitates a duck.

The person takes a feather from the ground and shows it for feathers. For bark, the persons says woof. For fur, the person puts on a fur coat, and then we have dog, which also has quack, feathers, bark and fur, and does dog-like things with these. So this allows us to abstract all of this data, and to use it in exactly the same way in each of these classes. Because each of these classes inherits AnimalActions, these methods become available in each of these other classes.

So when I create a duck object, here we have donald = Duck, that duck object will have these dictionary strings, Quaaaaak! The duck has gray and white feathers, and it will also have these action methods: quack, feathers, bark and fur. And so I can call duck.quack and I can call duck.feathers. The beauty of this is that I can also define a Person, john, and I can call in_the_forest, where it's expecting a duck, I can call it with john.

And so here we have an iterator. We are calling for o in ( donald, john, fido), so o will be sequentially don and john and fido. john is a person; john is not a duck. But in_the_forest(o) calls in the forest with john and in_the_forest expects a duck object, so it calls duck.quack and duck.feathers. Well, john implements - because john is a person, a person inherits AnimalActions, and it implements quack and feathers.

So this is called polymorphism. And let's see how this works. Remember, we are going to be calling in the doghouse and in the forest for each of these three objects: donald, john and fido. A Duck, a Person and a Dog. First, we'll call in the forest for each of them, and then we'll call in the doghouse for each of them. So I am going to run this. And we'll take a look at the results. In the forest, we have Quaaaak! And the duck has gray and white feathers.

So we are calling Quaaaak! and feathers for each of these. We are calling Quaaaak! and feathers also for the person, and it says the person imitates a duck, and the person takes the feather from the ground and shows it, and also for the dog, and then in the doghouse, the duck cannot bark, the duck has not fur, the person says woof! the person puts on a fur coat and then for the dog Arf! and the dog has white fur with black spots. So this is an example of polymorphism and inheritance in Python.

Now real quick, I want to also show you this example that's been rewritten as a Model-View-Controller. Model-View-Controller is a very common pattern in object-oriented programming, and I just want to show you that we can do exactly the same thing and actually separate it out a little bit further. In this case, we are not bothering to define the data for that which doesn't apply for each of these. So the dog doesn't quack, so I am not having a string for dog.

So this is the modeling, and we have the model for the duck and the person and the dog, and each of these inherits AnimalAction. And in this context, we don't really care about the implementation of AnimalActions, because all that matters is that we are modeling each of these types of objects. The view is the implementation of how do we view each of these things? So we have these four methods: bark, fur, quack and feathers. And they each call doAction with the word bark, fur, quack and feathers.

And then doAction is a local method, and you will notice it has this underscore. That doesn't make it special in Python, but it does tell the person who is reading it that this is something that's just going to be called internally. You are not going to want to call this on the object. You are just going to want to call it from inside of the object, or inside of the class. So this gets called from each of these methods. And if the action is not found in the strings, then it has a default that it does, where it says, this animal name has no action. And so that allows us to model these, and to actually expand them, and define more actions, and to not have to worry about doing a whole lot of extra modifications to the parent class. And then the controller is, of course, how we call all of this stuff, which is pretty much the same.

So this does exactly the same thing. If I go ahead and run this, we'll see that our result is exactly the same, but what we have here is a refined version of the same thing using a model, view and controller pattern. It's a contrived example, but this is commonly how we can do object-oriented programming in Python 3.

Find answers to the most frequently asked questions about Python 3 Essential Training .

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Q: The installation process for the PyDev Eclipse plug-in doesn’t work as described in the videos. What should I do?
A: The version of Eclipse used in the recording of Python 3 Essential Training was Eclipse 3.5.2 (Galileo SR2) and the version of PyDev was 1.5.7. Soon after recording, the Eclipse project was updated to version 3.6, called Helios. As of this writing, the current version of Eclipse is 3.6.1 (Helios SR1).
Around the same time as these updates were being released, the PyDev project was updated as well. As of this writing, the current version of PyDev is 1.6.2. If you are using these most recent versions, the procedure for installing Eclipse itself has not changed, but the process documented in the videos for installing PyDev may not work.

We will be releasing new versions of the videos soon, but the author has posted a document describing the new installation procedure at:
<a href="" target="blank></a>
Q: How do you install the pydev-interpreter in Eclipse Indigo on Mac OS X Lion? It's significantly different than what's shown in the video "Installing Python 3 and Eclipse for Mac."
A: Since this course was publishing, there have been upgrades to Mac OS X and Eclipse. In this scenario, after installing Python and Eclipse and the Pydev interpreter, there is a different directory to go to when modifying the preferences. As instructed in the movie (at around 6:40) restart Eclipse and then go to Eclipse > Preferences and drill down to Pydev > Interpreter - Python in the sidebar. Click New and in the Select interpreter dialog that opens, click Browse.

The Open dialog box will open, but does not appear to display your hard drive. You must press and hold the Command+Shift+Period keys. This will display all hidden files on your system. Navigate to the new path ~/Library/Frameworks/Python.framework/versions/3.2/bin/python 3.2 and click Open.

You should be able to proceed normally from there.
Q: When I try to install PyDev, it's not showing up in the Available Software window.
A:  This can happen if the site is down.
    You can manually download PyDev from the web site, or from my site here:

    Installation is simple. I've included instructions on my web site above.
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