Is-A Duck

Inheritance

Classes

class Duck:

    def looks_like(self):
        print("You see here a duck.")

    def swim(self):
        print("Paddle, paddle, paddle")

    def quack(self):
        print("Quack!")

    

Subclasses

Just like the parent class, except a little different

Reason #1: 

specialization

add * extend * override

>>> plain_dict = dict()
>>> print( plain_dict['no_such_key']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'no_such_key'
KeyError: 'no_such_key'

Example

defaultdict inherits from dict.

It's just like its parent, with one difference:

>>> import collections
>>> my_default_dict = collections.defaultdict(int)
>>> print( my_default_dict['no_such_key'])
0

Python dict() is a key-value map.

It raises an error if we access a key that doesn't exist.

Reason #2:

Substitution

Dr. Liskov

look like a duck

swim like a duck

quack like a duck

✓ 

A substitute duck must:

✓ 

✓ 

The 'L' in the SOLID principle

have duck DNA

✓ 

And in some languages,

Has-A Duck

Composition

import RubberDucky

class Ernie:

    def __init__(self):
        self.ducky = RubberDucky()

    def sing(self):
        print("Rubber ducky, you're the one")
        print( self.ducky.quack())
        print( self.ducky.quack())
        print("You make bath time lots of fun")

Ernie has a duck.

Ernie is not a duck.

Why?

• weaker relationship between elements
• more configurable
• less baggage
• my implementation is nobody else's business

Calls-A Duck

good old modules

often without classes

import json

duck = json.loads('{"name":"Daffy Duck", \
                    "species":"Loony Tune"}')

How to Choose

when to ask questions

Composition

• when possible
• when nothing calls the object from the outside
• when configurability is a plus

Inheritance

• when necessary
• when it's just like the parent with a small difference 

Modules

• all static methods or constants
• no instance data
• useful across many kinds of things
• "when your class has 2 methods, one of which is __init__"

Classes

• there are distinct instances with data
• inheritance is required

No Trogdors!

class Trogdor(Dragon, Man):

    def burninate(self):
        pass

    def shake_hands(self):
        pass

"Trogdor was a Man!

He was a Dragon Man!

Maybe he was just a dragon..."

Too Many Ancestors

Platypus

inherits bill, eggs from Duck

inherits fur, tail from Beaver

inherits poison spurs from Ninja

Lots of Imports