Variables

Instance variables

In the previous example, length is an instance variable of Square instances, which means that each instance of the class Square has its own copy of length, and the value stored in this copy may be different from the values stored in the length variable in other instances. In Python, you can create instance variables as needed by assigning them to the field of a class instance. If the variable does not already exist, it will be created automatically.

All uses of instance variables, both assignment and access, require explicit mention of the instance they contain, that is, instance.variable. A reference to a variable in itself is not a reference to an instance variable, but to a local variable in the executing method. This is different from C++ and Java, where instance variables are referenced in the same way as local function variables of the method. Python requires explicit mention of the contained instance here, and this enables a clear distinction between instance variables and local function variables.

Class variables

A class variable is a variable associated with a class, not an instance of a class, that can be accessed by all instances of the class. A class variable can be used to store some class-level information, such as how many instances of the class were created at a particular time. Python provides class variables, although using them requires a little more effort than in most other languages. You also need to be aware of an interaction between class and instance variables.

A class variable is created by an assignment in the class, but outside the __init__ function. After it is created, it can be seen by all instances of the class. You can use a class variable to make a value for pi accessible to all instances of the Circle class:

>>> class Circle:
...     pi = 3.14159
...     def __init__(self, diameter):
...         self.diameter = diameter
...     def circumference(self):
...         return self.diameter * Circle.pi
...

Once you have entered this definition, you can query pi with:

>>> Circle.pi
3.14159

Note

The class variable is linked to and contained within the class that defines it. You access Circle.pi in this example before any Circle instances have been created. It is obvious that Circle.pi exists independently of specific instances of the Circle class.

You can also access a class variable from a method of a class using the class name. You do this in the definition of Circle.circumference, where the circumference function contains a special reference to Circle.pi:

>>> c = Circle(3)
>>> c.circumference()
9.424769999999999

However, it is unpleasant that the class name Circle is used in the circumference method to address the class variable pi. You can avoid this by using the special __class__ attribute, which is available for all Python class instances. This attribute returns the class to which the instance belongs, for example:

>>> Circle
<class '__main__.Circle'>
>>> c.__class__
<class '__main__.Circle'>

The Circle class is internally represented by an abstract data structure, and this data structure is exactly what is obtained by the __class__ attribute of c, an instance of the Circle class. In this example, you can retrieve the value of Circle.pi from c without explicitly referring to the name of the Circle class:

>>> c.__class__.pi
3.14159

You can use this code internally in the circumference method to get rid of the explicit reference to the Circle class; replace Circle.pi with self.__class__.pi.

There is a little oddity about class variables that might confuse you if you are not aware of it.

Warning

If Python searches for an instance variable and does not find an instance variable with that name, it will search for and return the value in a class variable with the same name. Only if no matching class variable can be found does Python return an error. This can be used to efficiently implement default values for instance variables; however, this also easily leads to accidentally referring to an instance variable instead of a class variable without an error being reported.

First, you can refer to the variable c.pi, even though c has no associated instance variable called pi. Python first tries to find such an instance variable and only when it cannot find an instance variable does it look for a class variable pi in Circle:

>>> c1 = Circle(1)
>>> c1.pi
3.14159

If you now find that your specification for pi has been rounded too early and you want to replace it with a more precise specification, you might be inclined to change it as follows:

>>> c1.pi = 3.141592653589793
>>> c1.pi
3.141592653589793

However, you have now only added a new instance variable pi to c1. The class variable Circle.pi and all other instances derived from it still have only five decimal places:

>>> Circle.pi
3.14159
>>> c2 = Circle(2)
>>> c1.pi
3.14159