Counting Instances with Static Methods

Now, given these built-ins, here is the static method equivalent of this section’s instance-counting example—it marks the method as special, so it will never be passed an instance automatically:

class Spam:
    numInstances = 0                         # Use static method for class data
    def __init__(self):
        Spam.numInstances += 1
    def printNumInstances():
        print("Number of instances:", Spam.numInstances)
    printNumInstances = staticmethod(printNumInstances)

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Using the static method built-in, our code now allows the self-less method to be called through the class or any instance of it, in both Python 2.6 and 3.0:

>>> a = Spam()
>>> b = Spam()
>>> c = Spam()
>>> Spam.printNumInstances()                 # Call as simple function
Number of instances: 3
>>> a.printNumInstances()                    # Instance argument not passed
Number of instances: 3

Compared to simply moving printNumInstances outside the class, as prescribed earlier, this version requires an extra staticmethod call; however, it localizes the function name in the class scope (so it won’t clash with other names in the module), moves the function code closer to where it is used (inside the class statement), and allows subclasses to customize the static method with inheritance—a more convenient approach than importing functions from the files in which superclasses are coded. The following subclass and new testing session illustrate:

class Sub(Spam):
    def printNumInstances():                 # Override a static method
        print("Extra stuff...")              # But call back to original
        Spam.printNumInstances()
    printNumInstances = staticmethod(printNumInstances)

>>> a = Sub()
>>> b = Sub()
>>> a.printNumInstances()                    # Call from subclass instance
Extra stuff...
Number of instances: 2
>>> Sub.printNumInstances()                  # Call from subclass itself
Extra stuff...
Number of instances: 2
>>> Spam.printNumInstances()
Number of instances: 2

Moreover, classes can inherit the static method without redefining it—it is run without an instance, regardless of where it is defined in a class tree:

>>> class Other(Spam): pass                  # Inherit static method verbatim

>>> c = Other()
>>> c.printNumInstances()
Number of instances: 3