Appendix C: Classes

We are now going to bring what we have learnt about object oriented programming together as we define our own object type using classes.

Classes are how programmers define objects that make new objects. Think of them as object templates or factories.


The Koans are structured as classes with each koan as a method.

Defining & usage

Much like we defined functions lets define a class.

A file contains:

class Python():
    """ A class that represents a snake """

    def __init__(self, name, sex, age, length): = name = sex
        self.age = age
        self.length = length

    def move(self):
        print("{} moves".format(

    def eat(self):
        """ a snake gets longer when it eats """
        self.lenth = self.legnth + 1

    def starve(self):
        """ a snake shorter when it starves """
        # is there a bug here?
        self.lenth = self.legnth - 1

class object

Lets introspect the new type of object:

>>> from python import Python
>>> type(Python)
<class 'type'>
>>> dir(Python)
[ ... many methods ... ]


A class is like an object instance factory. Here our class makes snakes.

Implicitly it runs the __init__ function as defined on the class.


>>> john = Python('John', 'M', 15, 4)
>>> jane = Python('Jane', 'F', 4, 6)


>>> type(john)
<class 'python.Python'>
>>> dir(john)
[ ... many methods ... ]

Note we get move, starve, and eat which we defined, but we also get many methods others.


The other methods are those found when executing dir(object)


>>> Python.move
<function Python.move at 0x10f9b6840>
>>> john.move
<bound method Python.move of <python.Python object at 0x10fb04898>>

A function and a method are very similar. A function can stand alone, a method however is ‘bound’ to an object. When defined methods always take self as their first argument. It is thereby implicit when called.


The __str__ special method is called on an object when we pass it to the print function.

We decide that the semantics of printing a python is to show a visual representation of a snake using characters.

Added to definition in

class Python():

    def __init__(self, name, sex, age, length): = name

    def __str__(self):
        body = '=' * self.length
        return "{}>".format(body)


>>> from python import Python
>>> john = Python('John', 5)
>>> print(john)

special methods



Decide on the semantics of a python attacking another object.

Implement your decision by defining a new method.


Lets define another special method to exploit the nice syntax python gives us.

Decide on the semantics of ‘adding’ pythons together.

Implemnent by defining your __add__ method on the Python class.