Python:

Classes

What are classes?

Classes allow us to model 

complicated, real-life 

things in code

Until now, you've been using

simple data types

Numbers

Strings

To represent simple things

Numbers

length = 3price = 5.99count = 0

Strings

name = "George"poem = "Roses are Red\nViolets are Blue"option = "n"

Booleans

discount = Trueshow_help = False

But what if you need to 

represent something 

more complicated 

than a number?

(c)Tomo.Yun (www.yunphoto.net/en/)

Meet Spencer

(He's your goldfish)

We're going to put 

Spencer into a program

But how can you represent 

a Goldfish in Python?

First, think about fish

Not a specific fish,
but the idea of fish

What makes a fish a fish

rather than, say, a lobster?

Fish have scales

Breathe in water

Swim

Now think about Spencer

Spencer is a specific fish

We call Spencer an

"instance" of a fish

But they're all Fish

class = the idea of a fish

instances = the actual fish

Let's Code

(follow along!)

Define a basic Fish class

class Fish:    pass

     capital "F", please

     pass = "do nothing" (for now)

Make a fish

>> Fish()
<... Fish instance at ...> 

Make another

>> Fish()
<... Fish instance at ...> 

These are different fish!

>> a = Fish()
>> b = Fish()
>> a == b
False 

 

Our Fish class isn't very fish-y

class Fish:
    breathes_in_water = True
    skin = "scales" 

These variables exist on fish we create

>> myfish = Fish()
>> myfish.skin
"scales" >> if myfish.breathes_in_water:...    print "Glug glug!"
Glug glug!

You can also add functions

class Fish:
    def move(self, speed):
        print "swimming %s!" % speed

>> myfish = Fish()
>> myfish.move("fast")
swimming fast!

This gives your fish behavior

Wait a minute!

    ...    def move(self, speed):        ...
myfish.move("fast")

Python automatically puts

"myfish" into the "self" variable

class Fish:    def move(self, speed):        print self.name + " is moving " + speed + "!"
>> myfish = Fish()>> myfish.name = "Spencer">> myfish.move("fast")Spencer is moving fast!

To customize your fish, 

you can put any variables

on the instance

>> myfish = Fish()>> myfish.name = "Spencer">> myfish.color = "Gold"
>> myotherfish = Fish()>> myotherfish.name = "Susan">> myotherfish.color = "Blue"

Introducing __init__

__init__ is called whenever you
create a fish instance

class Fish:    def __init__(self):        print "Fish init!"
>> myfish = Fish()Fish init!
>> Fish()Fish init!

(the double underscores means it is "special")

Any parameters you pass into Fish()

will be passed to __init__()

class Fish:    def __init__(self, fish_name):        self.name = fish_name
>> myfish = Fish("Spencer")>> myfish.name"Spencer"

(See if you can add "color" to the init also)

Let's review a full Fish class

class Fish:    breathes_in_water = True    skin = "scales"
    def __init__(self, fish_name, fish_color):        self.name = fish_name        self.color = fish_color
    def move(self, speed):        print self.name + " is moving " + speed + "!"    >> spencer = Fish("Spencer", "Gold")>> print spencer.move("slowly")Spencer is moving slowly!

Now the fun starts

If we're building an aquarium

we're going to need more than

one kind of fish

is to store the species on Fish

class Fish:    def __init__(self, species):
        self.species = species
spencer = Fish("Goldfish")susan = Fish("Flounder")

But that is just a string.   

What if behavior is different?

Okay, then let's create a 

different class for Flounder

class Goldfish:    breathes_in_water = True    skin = "scales"    def move(self, speed):        print "Swimming upright %s!" % speed
class Flounder:    breathes_in_water = True    skin = "scales"    def move(self, speed):        print "Swimming sideways %s!" % speed

That seems tedious

What if they could share
the things that they
have in common?

Introducing parent classes

class Fish:    skin = "scales"
class Goldfish(Fish):        # <- Fish specified as parent    def move(self, speed):        print "Moving %s" % speed
>> spencer = Goldfish()>> spencer.skin             # <- So it has "skin" from Fish"scales"

Child classes can also
override parent's behavior

class Fish:    color = "Blue"    # Default fish color is Blue    skin = "scales"
class Flounder(Fish):    pass              # Inherits color = "Blue" from parent
class Goldfish(Fish):    color = "Gold"    # Override color for Goldfish
>> susan = Flounder()>> susan.color"Blue">> spencer = Goldfish()>> spencer.color"Gold"

We can test if a fish is
a certain type

class Fish:    pass
class Goldfish(Fish):    pass
class Flounder(Fish):    pass
>> spencer = Goldfish()>> isinstance(spencer, Fish)True>> isinstance(spencer, Goldfish)True>> isinstance(spencer, Flounder)False

Now that you know about
parent classes, always give
your top class a parent of
"object" (2.7 only)

class Fish(object):    pass
class Goldfish(Fish):    pass

object is the mother of all parent classes

Read this like

Goldfish is a more specific

Fish, which is a more specific

Object

Let's put some fish into
an aquarium

Try this by yourself before
continuing

Hint:

Aquarium class

class Aquarium(object):    fish = []    def __init__(self, fish):        self.fish = fish
class Fish(object):    color = "Blue"
    def __init__(self, name):        self.name = name
class Goldfish(Fish):    color = "Gold"
class Flounder(Fish):    pass
>> my_fish = [Goldfish("Spencer"), Goldfish("Vladimir"), Flounder("Susan")]>> my_aquarium = Aquarium(my_fish)>> for fish in my_aquarium.fish:...    print fish.name

Extra credit:  Feeding your fish

Implement a .feed() method
on both your aquarium class
and your fish class


Feeding your aquarium then
in turn feeds each of the fish
in it

Feeding your fish

class Aquarium(object):    ...    def feed(self, food):        for fish in self.fish:            fish.eat(food)
class Fish(object):    ...    def eat(self, food):        print self.name + " is eating " + food + "!"
class Goldfish(Fish):    pass
>> my_aquarium = Aquarium([Goldfish("Spencer"), Goldfish("Susan")])
>> my_aquarium.feed("flakes")Spencer is eating flakes!Susan is eating flakes!

That's all folks!

Questions?