Instance Variables and Class Variables

Variables declared at the “top level” in a class are attributes of the class–the same variable is common to all instances.  Class variables are most commonly referenced using dot notation with the class name, e.g., Horse.herd_size (see below). The variable can also be referenced through an instance, however, assigning this variable a value in this way creates a new copy of that variable local to the instance.  This feature in Python can be confusing, especially for students familiar with Java or C++.

For example, we can modify the Horse class to keep track of the herd size by introducing a class variable, herd_size:

class Horse:
    herd_size = 0

    def __init__(self, color="sorrel"):
        Horse.herd_size += 1
        self.color = color
        self.breed = "unknown"
        self.gender = None

After creating several instances of Horse, we can access the herd_size variable to see how many there are:

silver = Horse("white")
trigger = Horse("palomino")
wilbur = Horse("palomino")
buck = Horse("buckskin")
secretariate = Horse()


prints 5.

We can access the class variable through an instance, for example:


also prints 5. However, if we attempt to modify the variable through an instance, a new (local) copy of the variable is created instead:

silver.herd_size = 0;
sea_biscuit = Horse()

prints 0, 5, 6. The object referenced by silver now has a local copy of the herd_size variable, but the Horse class variable is unaffected, as are instances of Horse that have not created their own local copies.