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Understanding inheritance

From: Python 3 Essential Training

Video: Understanding inheritance

In object-oriented programming, inheritance is when one class inherits the properties of another class. The class that is being inherited from is often called a base class or a parent class. Let's take a look at how this is done in Python. We'll start by making a working copy of classes.py, classes-working.py, and we'll open our working copy. You notice that we have this Duck class here. We're going to create another class. We'll call this one Animal.

Understanding inheritance

In object-oriented programming, inheritance is when one class inherits the properties of another class. The class that is being inherited from is often called a base class or a parent class. Let's take a look at how this is done in Python. We'll start by making a working copy of classes.py, classes-working.py, and we'll open our working copy. You notice that we have this Duck class here. We're going to create another class. We'll call this one Animal.

So the Animal class will have methods in it that will be inherited by the Duck class and perhaps other animals as well. We'll start by saying that an animal makes a sound, so we'll call talk and that will print 'I have something to say!', and say that the animal moves around. We'll call that walk, print. 'Hey! I' 'm walkin'' here!' And then an animal is covered with something, usually fur or feathers or something. We'll call that clothes.

'I have nice clothes'. [00:01:22.02 All right. So there is our animal. Now, Duck can inherit all of these methods from Animal, in fact all of its attributes, by simply saying Animal here. Putting in a parenthesis in the definition of Duck. Now, Duck is said to be, is an animal. Duck is a animal. Object-oriented speak for saying that the Duck inherits the properties of Animal.

So Duck will continue to work the same way that it did before. If we save this and we run it, we don't really see any difference in how it works. Except that now it has access to these other properties. I can say donald.clothes and save that and run it. And we see donald says he has nice clothes. You'll notice that walk printed "Walks like a duck." It did not print "Hey! I'm walkin' here!" That's because walk in Duck overrides walk in Animal.

The way that works is that Duck inherits Animal and then Duck defines a method called walk, so it uses this one instead of the one in its parent. If we wanted to say incorporate the one from its parent, we can do that with the super function. This is a built-in function which accesses the parent class and just say, super().walk() like that, and now donald will do both. Save that and run it, and he says, "Hey! Im walkin here!" And he says, "Walks like a duck." So this is how you can access the class in the parent, even though you're overriding it in the class that inherits from the parent.

So what makes this useful is that now we've defined an animal base class and now if we want to have another animal, it's very easy for us to do that. We can say class Dog and it inherits from Animal, and that's really all that we need to define. We can just say pass here and we can say fido = Dog(), and fido.walk(), and we have a working dog.

Save that and run it. And there we have, "Hey! I'm walkin' here!" And that's from fido. So obviously a dog, instead of having clothes, he is got fur. So we can say, def clothes, print, 'I have brown and white fur'. And then down here if we say fido.clothes(), save that and run it, and it says, "I have brown and white fur." But what we've done is we've created a base class that perhaps has all the properties that are going to be common to the classes that we're creating and then we simply inherit that and that makes our code very reusable.

You'll see this technique used a lot. In fact, in an object-oriented language like Python, virtually all of the classes at some level are inheriting form other classes. You have two classes like the tuple and the list and they do a lot of common similar things. In fact, strings do as well. And so those are all inheriting from some common classes and simply building upon that and overwriting some methods and implementing some and not implementing others.

And that allows them to reuse a lot of code and it also allows them to operate in a lot of the same contexts as each other. So this is inheritance and this is how you do inheritance in Python.

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This video is part of

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Python 3 Essential Training

87 video lessons · 39618 viewers

Bill Weinman
Author

 
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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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