Session Six: Intro to Object Oriented Programming

Classes, instances, attributes, and subclassing

Review/Questions

Review of Previous Class

  • Argument Passing: *args, **kwargs
  • comprehensions
  • lambda
  • Solutions to the FizzBuzz problem.

Homework review

  • LBYL vs. EAFP

http://stackoverflow.com/questions/404795/lbyl-vs-eafp-in-java

Other Homework Questions?

Review of Survey Feedback

  • Introductory readings
  • More up-front explanation in class
  • More connection between homework and in-class exercises
  • Faster homework grading

Questions

  • How will this prepare me for the dev accelerator?
  • What about independent projects for my software portfolio?
  • Do we have to worry about proper Git / GitHub technique?

Resources

Beginner-Friendly Textbooks

Portfolio Projects, Building Community

http://newcoder.io/

Calling Twitter APIs (thanks @mhazani!)

Preparation for Dev Accelerator Code Challenge

Django Resources

Object Oriented Programming

Object-oriented programming narrows the “semantic gap”.

You can model real world objects with software objects.

We’ll talk more about Python implementation than OO design/strengths/weaknesses

More Detailed Reading:

Dive Into Python, 5.3-5.5 on Classes `Learn Python the Hard Way <

Object Oriented Programming

Is Python a “True” Object-Oriented Language?

(Doesn’t support full encapsulation, doesn’t require classes, etc...)

I don’t Care!

Good software design is about code re-use, clean separation of concerns, refactorability, testability, etc...

OO can help with all that, but:
  • It doesn’t guarantee it
  • It can get in the way

Python is a Dynamic Language

That clashes with “pure” OO

Think in terms of what makes sense for your project – not any one paradigm of software design.

So what is “object oriented programming”?

Objects can be thought of as wrapping their data within a set of functions designed to ensure that the data are used appropriately, and to assist in that use

http://en.wikipedia.org/wiki/Object-oriented_programming

Even simpler:

“Objects are data and the functions that act on them in one place.”

This is the core of “encapsulation”

In Python: just another namespace.

The OO buzzwords:

  • data abstraction
  • encapsulation
  • modularity
  • polymorphism
  • inheritance

Python does all of this, though it doesn’t enforce them.

“OO languages” give you some handy tools to make it easier (and safer):

  • polymorphism (duck typing gives you this anyway)
  • inheritance

OO has been the dominant model for the past couple decades

You will need to use it:

  • It’s a good idea for a lot of problems
  • You’ll need to work with OO packages

(Even a fair bit of the standard library is Object Oriented)

class
A category of objects: particular data and behavior: A “circle” (same as a type in python)
instance
A particular object of a class: a specific circle
object
The general case of a instance – really any value (in Python anyway)
attribute
Something that belongs to an object (or class): generally thought of as a variable, or single object, as opposed to a ...
method
A function that belongs to a class

Note that in python, functions are first class objects, so a method is an attribute

Python Classes

The class statement

class creates a new type object:

In [4]: class C(object):
   ...:     pass
   ...:
In [5]: type(C)
Out[5]: type

A class is a type – interesting!

It is created when the statement is run – much like def

You don’t have to subclass from object, but you should

(note on “new style” classes)

Python Classes

About the simplest class you can write

>>> class Point(object):
...     x = 1
...     y = 2
>>> Point
<class __main__.Point at 0x2bf928>
>>> Point.x
1
>>> p = Point()
>>> p
<__main__.Point instance at 0x2de918>
>>> p.x
1

Basic Structure of a real class:

class Point(object):
    # everything defined in here is in the class namespace

    def __init__(self, x, y):
        # everything attached to self is in the instance namespace
        self.x = x
        self.y = y

## create an instance of the class
p = Point(3,4)

## access the attributes
print "p.x is:", p.x
print "p.y is:", p.y

see: Examples/Session06/simple_classes.py

The __init__ special method is called when a new instance of a class is created.

You can use it to do any set-up you need

class Point(object):
    def __init__(self, x, y):
        self.x = x
        self.y = y

It gets the arguments passed when you call the class object:

Point(x, y)

What is this self thing?

The instance of the class is passed as the first parameter for every method.

Using self is only a convention – but you DO want to use it.

class Point(object):
    def a_function(self, x, y):
...

Does this look familiar from C-style procedural programming?

Anything assigned to a self.<xyz> attribute is kept in the instance name space – self is the instance.

That’s where all the instance-specific data is.

class Point(object):
    size = 4
    color= "red"
    def __init__(self, x, y):
        self.x = x
        self.y = y

Anything assigned in the class scope is a class attribute

Every instance of the class shares the same one.

Note: the methods defined by def are class attributes as well.

The class is one namespace, the instance is another.

class Point(object):
    size = 4
    color= "red"
...
    def get_color():
        return self.color
>>> p3.get_color()
 'red'

Class attributes are accessed with self also.

Typical methods:

class Circle(object):
    color = "red"

    def __init__(self, diameter):
        self.diameter = diameter

    def grow(self, factor=2):
        self.diameter = self.diameter * factor

Methods take some parameters, manipulate the attributes in self.

They may or may not return something useful.

...
    def grow(self, factor=2):
        self.diameter = self.diameter * factor
...
In [205]: C = Circle(5)
In [206]: C.grow(2,3)

TypeError: grow() takes at most 2 arguments (3 given)

Huh???? I only gave 2

self is implicitly passed in for you by python.

(demo of bound vs. unbound methods)

Using self explicitly like this can seem a bit confusing

But like most of Python’s quirks, there’s a rationale behind it

Our BDFL has made the decision that self will stay, and written extensively about why:

http://neopythonic.blogspot.com/2008/10/why-explicit-self-has-to-stay.html

LAB / Homework

Let’s say you need to render some html..

The goal is to build a set of classes that render an html page.

Examples/Session06/sample_html.html

We’ll start with a single class, then add some sub-classes to specialize the behavior

Details in:

homework_html_renderer

Let’s see if we can do step 1. in class...

Subclassing/Inheritance

Inheritance

In object-oriented programming (OOP), inheritance is a way to reuse code of existing objects, or to establish a subtype from an existing object.

Objects are defined by classes, classes can inherit attributes and behavior from pre-existing classes called base classes or super classes.

The resulting classes are known as derived classes or subclasses.

(http://en.wikipedia.org/wiki/Inheritance_%28object-oriented_programming%29)

Subclassing

A subclass “inherits” all the attributes (methods, etc) of the parent class.

You can then change (“override”) some or all of the attributes to change the behavior.

You can also add new attributes to extend the behavior.

The simplest subclass in Python:

class A_subclass(The_superclass):
    pass

A_subclass now has exactly the same behavior as The_superclass

NOTE: when we put object in there, it means we are deriving from object – getting core functionality of all objects.

Overriding attributes

Overriding is as simple as creating a new attribute with the same name:

class Circle(object):
    color = "red"

...

class NewCircle(Circle):
    color = "blue"
>>> nc = NewCircle
>>> print nc.color
blue

all the self instances will have the new attribute.

Overriding methods

Same thing, but with methods (remember, a method is an attribute in python)

class Circle(object):
...
    def grow(self, factor=2):
        """grows the circle's diameter by factor"""
        self.diameter = self.diameter * factor
...

class NewCircle(Circle):
...
    def grow(self, factor=2):
        """grows the area by factor..."""
        self.diameter = self.diameter * math.sqrt(2)

all the instances will have the new method

A Program Design Suggestion:

whenever you override a method, the interface of the new method should be the same as the old. It should take the same parameters, return the same type, and obey the same preconditions and postconditions.

A Program Design Suggestion

If you obey this rule, you will find that any function designed to work with an instance of a superclass, like a Deck, will also work with instances of subclasses like a Hand or PokerHand. If you violate this rule, your code will collapse like (sorry) a house of cards.

—[ThinkPython 18.10]

( Demo of class vs. instance attributes )

More on Subclassing

Overriding __init__

Wanting or needing to override __init__ is very common

You often need to call the super class __init__ as well

Think “everything the parent does, plus this stuff too”

class Circle(object):
    color = "red"
    def __init__(self, diameter):
        self.diameter = diameter
...
class CircleR(Circle):
    def __init__(self, radius):
        diameter = radius*2
        Circle.__init__(self, diameter)

exception to: “don’t change the method signature” rule.

More subclassing

You can also call the superclass’ other methods:

class Circle(object):
...
    def get_area(self, diameter):
        return math.pi * (diameter/2.0)**2


class CircleR2(Circle):
...
    def get_area(self):
        return Circle.get_area(self, self.radius*2)

There is nothing special about __init__ except that it gets called automatically when you instantiate an instance.

When to Subclass

“Is a” relationship: Subclass/inheritance

“Has a” relationship: Composition

“Is a” vs “Has a”

You may have a class that needs to accumulate an arbitrary number of objects.

A list can do that – so should you subclass list?

Ask yourself:

Is your class a list (with some extra functionality)?

or

– Does you class have a list?

You only want to subclass list if your class could be used anywhere a list can be used.

Attribute resolution order

When you access an attribute:

An_Instance.something

Python looks for it in this order:

  • Is it an instance attribute?
  • Is it a class attribute?
  • Is it a superclass attribute?
  • Is it a super-superclass attribute?
  • ...

What are Python classes, really?

Putting aside the OO theory...

Python classes are:

  • Namespaces
    • One for the class object
    • One for each instance
  • Attribute resolution order
  • Auto tacking-on of self when methods are called

That’s about it – really!

Type-Based dispatch

You’ll see code that looks like this:

if isinstance(other, A_Class):
    Do_something_with_other
else:
    Do_something_else

Usually better to use “duck typing” (polymorphism)

But when it’s called for:

  • isinstance()
  • issubclass()

GvR: “Five Minute Multi- methods in Python”:

http://www.artima.com/weblogs/viewpost.jsp?thread=101605

http://www.python.org/getit/releases/2.3/mro/

http://python-history.blogspot.com/2010/06/method-resolution-order.html

Wrap Up

Thinking OO in Python:

Think about what makes sense for your code:

  • Code re-use
  • Clean APIs
  • ...

Don’t be a slave to what OO is supposed to look like.

Let OO work for you, not create work for you

The Art of Subclassing: Raymond Hettinger

http://pyvideo.org/video/879/the-art-of-subclassing

“classes are for code re-use – not creating taxonomies”

Stop Writing Classes: Jack Diederich

http://pyvideo.org/video/880/stop-writing-classes

“If your class has only two methods and one of them is __init__, you don’t need a class”

Homework

Task 17: HTML Renderer

Build an html rendering system:

homework_html_renderer

You will build an html generator, using:

  • A Base Class with a couple methods
  • Subclasses overriding class attributes
  • Subclasses overriding a method
  • Subclasses overriding the __init__

These are the core OO approaches

Create a directory called session06 in your student directory. Create a branch in your local repo called task17 and switch to it (git checkout -b task17).

Add your files to that branch, commit frequently, and push to it as you work, writing good commit messages. Then create a pull request to the main class repo, titled Task 17 pull request from Your Name where you should substitute your name for Your Name.

Task 18: Investigate Session 7

Read through the Session 7 slides.

http://codefellows.github.io/sea-c34-python/session07.html

There are five sections. For each one, come up with one question.

  • Testing (1 question)
  • Multiple Inheritance (1 question)
  • Properties (1 question)
  • Class and Static Methods (1 question)
  • Special (Magic) Methods (1 question)

Write some Python code to answer these questions, one function per question.

For each function, write a good docstring describing what question you are trying to answer.

Put the functions in four separate modules (files) called testing.py, multiple.py, properties.py, static.py, and special.py in the session06 subdirectory of your student directory.

That is, you should have seven questions, and seven functions, total, spread out across three files.

You may use everything you’ve learned so far as needed (including lists, tuples, slicing, iteration, functions, booleans, printing, modules, assertions, dictionaries, sets, exceptions, file reading/writing, paths, lambdas, keyword/variable arguments, comprehensions, and object-oriented programming).

Create a branch in your local repo called task18 and switch to it (git checkout task18).

Add your files to that branch, commit and push, then create a pull request to the main class repo, titled Task 18 pull request from Your Name where you should substitute your name for Your Name.

Finally, submit your assignment in Canvas by giving the URL of the pull request.