Python Classes

The class statement

class creates a new type object:

In [4]: class C:
    pass
   ...:
In [5]: type(C)
Out[5]: type

A class is a type – interesting!

It is created when the statement is run – much like def

A simple class

About the simplest class you can write

>>> class Point:
...     x = 1
...     y = 2
>>> Point
<class __main__.Point at 0x2bf928>
>>> Point.x
1
>>> p = Point()
>>> p
<__main__.Point instance at 0x2de918>
>>> p.x
1

Basic Structure of a class

class Point:
# everything defined in here is in the class namespace

    def __init__(self, x, y):
        self.x = x
        self.y = y

## create an instance of the class
p = Point(3,4)

## access the attributes
print("p.x is:", p.x)
print("p.y is:", p.y)

see: Examples/Session07/simple_classes.py

The Initializer

The __init__ special method is called when a new instance of a class is created.

You can use it to do any set-up you need

class Point(object):
    def __init__(self, x, y):
        self.x = x
        self.y = y

It gets the arguments passed when you call the class object:

Point(x, y)

Self

What is this self thing?

The instance of the class is passed as the first parameter for every method.

“self” is only a convention – but you DO want to use it.

class Point:
    def a_function(self, x, y):
...

Does this look familiar from C-style procedural programming?

Anything assigned to a self. attribute is kept in the instance name space – self is the instance.

That’s where all the instance-specific data is.

class Point(object):
    size = 4
    color= "red"
    def __init__(self, x, y):
        self.x = x
        self.y = y

Class Attributes

Anything assigned in the class scope is a class attribute – every instance of the class shares the same one.

Note: the methods defined by def are class attributes as well.

The class is one namespace, the instance is another.

class Point:
    size = 4
    color= "red"
...
    def get_color():
        return self.color
>>> p3.get_color()
 'red'

class attributes are accessed with self also.

Typical methods

class Circle:
    color = "red"

    def __init__(self, diameter):
        self.diameter = diameter

    def grow(self, factor=2):
        self.diameter = self.diameter * factor

Methods take some parameters, manipulate the attributes in self.

They may or may not return something useful.

Arity Gotcha

...
    def grow(self, factor=2):
        self.diameter = self.diameter * factor
...
In [205]: C = Circle(5)
In [206]: C.grow(2,3)

TypeError: grow() takes at most 2 arguments (3 given)

Huh???? I only gave 2

self is implicitly passed in for you by python.

Functions (methods) are First Class

Note that in python, functions are first class objects, so a method is an attribute