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Python 3 Essential Training
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Using methods


From:

Python 3 Essential Training

with Bill Weinman

Video: Using methods

When you're creating objects and classes in Python, you're going to want to get them some functionality. That functionality is usually created with methods. So let's go ahead and make a working copy of classes.py. We'll call this classes-working.py. We'll open up that working copy. You can see that we have a class called Duck and it already has couple of methods in it, quack and walk. Those get called down here. So donald gets created as class Duck. So this is an object with that class and the object donald.
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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
      29s

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Python 3 Essential Training
6h 36m Beginner Jul 29, 2010

Viewers: in countries Watching now:

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
Subjects:
Developer Web Programming Languages Web Development
Software:
Python
Author:
Bill Weinman

Using methods

When you're creating objects and classes in Python, you're going to want to get them some functionality. That functionality is usually created with methods. So let's go ahead and make a working copy of classes.py. We'll call this classes-working.py. We'll open up that working copy. You can see that we have a class called Duck and it already has couple of methods in it, quack and walk. Those get called down here. So donald gets created as class Duck. So this is an object with that class and the object donald.

We're calling the quack method on the object donald and we're calling the walk method on the object donald. The calls are done using this dot operator, which is used to reference an attribute of the object. In this case, the attribute is a method. So it's called with the parenthesis and that calls it as you would call a function. These are actually functions. It's just that these are functions that are attributes of an object. That's what makes the methods.

So we'll go ahead and we'll run this, and you'll see that the object gets created and the methods get called. It prints out Quaaack! and it prints out Walks like a duck. Everything is working like we expected it to. So methods are actually functions. You'll see that they're defined just like you define functions in Python. The difference is these are indented under the class Duck. So that makes them methods of the class Duck. You'll also notice that the first argument to these functions is this word self. That is a reference to the object. Not the class, but the object.

In other words, when quack gets called on the donald object, then the donald object gets passed to the quack method. So the quack has a way of referring to object attributes. You'll also notice that nothing is getting passed inside of these parentheses. It's as if donald was passed inside those parentheses, but you don't actually put that there. That happens automatically by virtue of the dot operator. When quack gets called as a method of donald, then donald gets passed magically inside those parentheses, so that it's the first argument to the method.

There's a special type of a method that I want to talk about which is called a constructor. This gets called every time you create an object based on this class. The constructor in Python is created by naming a method with two underscores and the word init and two more underscores at the end. This creates a constructor method. So I'm just going to print('constructor') here. And we'll go ahead and we'll save this and we'll run it. You'll see that constructor gets printed at the beginning.

So when we run this, the first thing we do is we create the donald object and that's where the constructor gets called. In fact, one of the most common purposes for a constructor is to initialize some data. So if I put in the number 47 here and then I pass in that value in the constructor, I'll say value here, instead of printing constructor here, I can save this value, saying self._v = value.

What this does is this creates a local variable that is an attribute of the object. In this case, the donald object, but if I had different objects with different names, it would be attributes of those objects. We'll see how valuable this is in a moment. Then I can use that value in the methods. For example, I can print it out here. I can print it out here. So now I'm initializing this variable to the number 47.

Each time I use it, I'm going to go ahead and print it out. So we'll save this and run it. Now we see that we have a 47 here and a 47 there. If I were to change this to something else, save it and run it. We have 52 here and 52 there. If I create a different object with a different number in it and go ahead and call it, now when I save that and run it, then we have the donald ducks and we have the frank ducks.

So the value here is that this self._v is actually attached to the object, not to the class. This is what's called encapsulation in object oriented programming, because that value is a part of the object. So I can have different objects with different values. I could actually have all kinds of different data associated with those objects. They could be opening different databases, they could be accessing different files, they could be keeping their place in these databases and in these files.

This is the power of encapsulation in object oriented programming. We'll see a lot of examples of that as we move forward, but for now what's important to know is that all this happens to this self variable right there and right there. When the object calls a method that self variable gets passed, and that's a reference to the object. All the things that are attached to the object, its other methods, its attributes, its data, is all carried there. So here we've defined methods in our object. These methods are defined in exactly the same way that we define a function in Python except that its first argument is always self and that argument doesn't get passed explicitly.

It's passed implicitly through the dot operator. Then we have a special method called init which is used as a constructor. We can use that to set up data. We can use it to open databases. We can use it for all kinds of purposes. The point of it is to initialize whatever needs to get initialized when you create the object based on this class. So that's how you can create a constructor and that's how you create methods in classes that will be used in objects in Python.

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="http://python.bw.org/pydev-installation/" target="blank>http://python.bw.org/pydev-installation/</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 pydev.org/updates site is down.
 
    You can manually download PyDev from the pydev.org web site, or from my site here:
 
        http://python.bw.org/
 
    Installation is simple. I've included instructions on my web site above.
 
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