Python Class vs Instance Variables

Python class variables offer a key tool for building libraries with DSL type syntax, as well as data shared between all instances of the class. This page explains how Python class variables work and gives some examples of usage.

• Class & Instance Variables
  • Instances vs the class object
  • Instance Variables
  • Class Variables
• Using Class and Instance Variables

Class & Instance Variables

Instances vs the class object

In object oriented programming there is the concept of a class, and the definition of that class is the ‘blueprint’ for objects of that class.  The class definition is used to create objects which are instances of the class. In Python, for each class, there is an additional object for the class itself.  This object for the class itself, is an object of the class ‘type’. So if there is a class ‘Foo’ with two instanced objects ‘a’ and ‘b’, created by a = Foo() and b = Foo(), this creates objects a and b of class Foo. In Python, the code declaring the class Foo does the equivalent of, for example, Foo = type(). This Foo object can be manipulated at run time, the object can be inspected to discover things about the class, and the object can even be changed to alter the class itself at run time.

Instance Variables

Consider the following  code  as run in Idle (Python 3.6):

class Foo:
    def __init__(self):
         self.var = 1
>>> a = Foo()
>>> type(a)
'class: __man__'.Foo
>>type(Foo)
'class: type'
>>'var' in a.__dict__
True
>>>var in Foo.__dict__
False
>>> a.var
1
>>> Foo.var
Traceback (most recent call last):
  File "", line 1, in
AttributeError: type object 'Foo' has no attribute 'var'

Note the results of type(a) compared to type(Foo). 'var' appears in the a.__dict__ , but not in the Foo.__dict__ within Foo. Further, a.var gives a value of 1 while Foo.var returns an error.

This is all quite straightforward, and is as would be expected.

Class Variables

Now consider this code as run in  Idle that has a class variable in Idle (Python 3.6):

class Foo:
    var = 1
>>> a = Foo()
>>'var' in a.__dict__
False
>>>'var' in Foo.__dict__
True

>>> Foo.var
1
>>> a.var
1
>>> a.var = 2
>>> a.var
2
>>> Foo.var
1
>>> a.__class__.var
1

All as would be expected, the __dict__ results are reversed, this time the class Foo initially has var, and the instance a does not.

But even though a does not have a var attribute,  a.var returns 1, because when there is no instance variable var, Python will return a class level variable if one is present with the same name.

However, assignment does not fall back to class level variables, so setting a.var = 2 actually creates an instance variable var and reviewing the __dict__ data now reveals  both that the class level object and instance each have var. Once an instance variable is present, the class level variable is hidden from access using a.var which will now access the instance variable. In this way, code can add an instance variable replacing the value of class variable which provides what can be effectively a default value until the instance value is set.

Simple Usage Example

Consider the following Python class:

class XYPoint:

    scale = 2.0
    serial_no = 0

    def __init__(self, x, y):
        self.serial_no = self.serial_no + 1
        self.__class__.serial_no = self.serial_no
        self.x = x
        self.y = y

    def scaled_vector(self):
        vector = (self.x**2 + self.y**2)**.5
        return vector * self.scale

   def set_scale(self, scale):
        self.__class__.scale = scale

    def __repr__(self):
        return f"XYPoint#{self.serial_no}(x={self.x},y={self.y})"

The class encloses an (x,y) coordinate, and has a method scaled_vector() which calculates a vector (using Pythagoras theorem) from that coordinate, and then scales the vector from the class variable scale.

If the class level variable scale is changed, automatically all XYPoint objects will return vectors using the new scale.

Being a class variable, scale can be read as self.scale but must be set as in the set_scale() method by self.__class__.scale = scale.

The other use case illustrated is the instance counter serial_no, which provides each instance of XYPoint a unique, incrementing serial_no.