Gentle Object Oriented Programming in Python

Nicholas H.Tollervey / @ntoll
Naomi Ceder / @NaomiCeder

We model the world with language.

Language includes:

Naming types of things (cats, dogs, cows)
Naming specific things (Garfield, Snoopy, Buttercup)
Describing actions (run, walk, smile, love, hate)
Attributing qualities (red, furry, hot, scary)

Nouns, proper nouns, verbs and adjectives.

Computers need to model the world too!

Like this:

(Python Refresher)


    def moo(name, message='MOO!'):
        """
        A function to return a greeting.
        """
        return '{} says, {}'.format(name, message)

A function takes arguments and returns a result, like this:


    >>> moo('Buttercup')
    'Buttercup says, MOO!'
    >>> moo('Daisy', 'MooOOOOooooo!')
    'Daisy says, MooOOOOooooo!'

Growing the Herd...

If we have 4 cows...


    >>> a_cow = 'Buttercup'
    >>> another_cow = 'Daisy'
    >>> yet_another_cow = 'Bessie'
    >>> yet_another_other_cow = 'Clover'
    >>> a_cow_breed = 'Friesian'
    >>> another_cow_breed = 'Galloway'
    >>> yet_another_cow_breed = 'Guernsey'
    >>> yet_another_other_cow_breed = 'Dexter'

But what if we have 50 cows? 500?

lists help...


    >>> cow_names = ['Buttercup', 'Daisy', 'Bessie', 'Clover']
    >>> cow_breeds = ['Friesian', 'Galloway', 'Guernsey', 'Dexter']

What could possibly go wrong with this?

Well, this...


    >>> del cow_names[1]
    >>> cow_names
    >>>  ['Buttercup', 'Bessie', 'Clover']
    >>> del cow_breeds[2]
    >>> cow_breeds
    >>> ['Friesian', 'Galloway','Dexter']

Uh-oh...

(what's gone wrong here..?)

Basic OOP
Concepts

A class defines a type of thing

An object is an instance of a class

A method specifies a behaviour of an object

An attribute defines the quality of an object

Crib sheet

Class = Noun
Object = Proper noun
Method = Verb
Attribute = Adjective

(This is not entirely accurate but is close enough for today)

What about Python?

Python Classes


    class Cow:
        """
        A pythonic cow. :-)
        """

        def __init__(self, name='Cow', breed=None):
            """
            Called when the class is instantiated.
            """
            self.name = name
            self.breed = breed

        def moo(self, message='MOO!'):
            """
            A bovine greeting method (see earlier slide).
            """
            return '{} says, {}'.format(self.name, message)

Created in the file cow.py

A Herd of Pythonic Cows


    >>> from cow import Cow
    >>> a_cow = Cow('Buttercup', 'Friesian')
    >>> a_cow.name
    'Buttercup'
    >>> a_cow.breed
    'Friesian'
    >>> a_cow.moo()
    'Buttercup says, MOO!'
    >>> another_cow = Cow('Daisy', 'Galloway')
    >>> another_cow.name
    'Daisy'
    >>> another_cow.breed
    'Galloway'
    >>> another_cow.moo('MooOOOOooooo!')
    'Daisy says, MooOOOOooooo!'

a_cow and another_cow are instances of the Cow class.

Parrots!

Your task...

Create a "Parrot" class.
You must be able to give a parrot a name and catch-phrase (e.g. "Pieces of eight...").
Instantiate a Parrot with the name "Polly" and a catch-phrase of your choosing.
Call Polly's "speak" method so Polly returns her catch-phrase.
Bonus points for creativity and surprises!

Crib sheet...


    class Cow:
        """
        A pythonic cow. :-)
        """

        def __init__(self, name='Cow', breed=None):
            """
            Called when the class is instantiated.
            """
            self.name = name
            self.breed = breed

        def moo(self, message='MOO!'):
            """
            A bovine greeting method (see earlier slide).
            """
            return '{} says, {}'.format(self.name, message)

    my_cow = Cow('Buttercup', 'Galloway')
    print(my_cow.moo())

Saved in the file cow.py and run with the command: python cow.py