What are classes?

Classes allow us to model 

complicated, real-life 

things in code

Until now, you've been using

simple data types




To represent simple things


length = 3price = 5.99count = 0


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

discount = Trueshow_help = False

But what if you need to 

represent something 

more complicated 

than a number?

(c)Tomo.Yun (

Meet Spencer

(He's your goldfish)

We're going to put 

Spencer into a program

Let's make a virtual 

aquarium for him

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?

We define these fish-y

characteristics as a "class"

Fish have scales

Breathe in water


Now think about Spencer

Spencer is a specific fish

He is your fish,

swimming in your fishbowl

We call Spencer an

"instance" of a fish

There are other instances

of fish (for example, your 

sister's goldfish)

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

Our Fish class isn't very fish-y

Let's add some fishy-ness

class Fish:
    breathes_in_water = True
    skin = "scales" 

These variables exist on fish we create

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

You can also add functions

(a.k.a methods)

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!

(You might say)

.move() requires 2 parameters, 

and you only passed 1!

    ...    def move(self, speed):        ...

Python automatically puts

"myfish" into the "self" variable

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

To customize your fish, 

you can put any variables

on the instance

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

But that takes too much typing

(There's a shortcut)

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): = fish_name
>> myfish = Fish("Spencer")>>"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): = fish_name self.color = fish_color
def move(self, speed): print + " 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

One way you might think

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

Plus, there's so much duplicated code!

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()>> # <- So it has "skin" from Fish"scales"

Classes will "inherit" all of 
their parent's behavior
and properties

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 being the most general thing we can arrive at)

Let's put some fish into
an aquarium

Try this by yourself before


Make another class called Aquarium that holds
instances of fish, and assign some fish instances to it

Aquarium class

class Aquarium(object):    fish = []    def __init__(self, fish): = fish
class Fish(object): color = "Blue"
def __init__(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 print

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  
class Fish(object): ... def eat(self, food): print + " 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!


Classes in Python

By Jason Myers

Classes in Python

A brief tutorial on Python classes, initially used as a supplement for

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