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Static Method Alternatives

Short of marking a self-less method as special, there are a few different coding structures that can be tried. If you want to call functions that access class members without an instance, perhaps the simplest idea is to just make them simple functions outside the class, not class methods. This way, an instance isn’t expected in the call. For example, the following mutation of spam.py works the same in Python 3.0 and 2.6 (albeit displaying extra parentheses in 2.6 for its print statement):

def printNumInstances():
    print("Number of instances created: ", Spam.numInstances)

class Spam:
    numInstances = 0
    def __init__(self):
        Spam.numInstances = Spam.numInstances + 1

>>> import spam
>>> a = spam.Spam()
>>> b = spam.Spam()
>>> c = spam.Spam()
>>> spam.printNumInstances()           # But function may be too far removed
Number of instances created:  3        # And cannot be changed via inheritance
>>> spam.Spam.numInstances
3

Because the class name is accessible to the simple function as a global variable, this works fine. Also, note that the name of the function becomes global, but only to this single module; it will not clash with names in other files of the program.

Prior to static methods in Python, this structure was the general prescription. Because Python already provides modules as a namespace-partitioning tool, one could argue that there’s not typically any need to package functions in classes unless they implement object behavior. Simple functions within modules like the one here do much of what instance-less class methods could, and are already associated with the class because they live in the same module.

Unfortunately, this approach is still less than ideal. For one thing, it adds to this file’s scope an extra name that is used only for processing a single class. For another, the function is much less directly associated with the class; in fact, its definition could be hundreds of lines away. Perhaps worse, simple functions like this cannot be customized by inheritance, since they live outside a class’s namespace: subclasses cannot directly replace or extend such a function by redefining it.

We might try to make this example work in a version-neutral way by using a normal method and always calling it through (or with) an instance, as usual:

class Spam:
    numInstances = 0
    def __init__(self):
        Spam.numInstances = Spam.numInstances + 1
    def printNumInstances(self):
        print("Number of instances created: ", Spam.numInstances)

>>> from spam import Spam
>>> a, b, c = Spam(), Spam(), Spam()
>>> a.printNumInstances()
Number of instances created:  3
>>> Spam.printNumInstances(a)
Number of instances created:  3
>>> Spam().printNumInstances()         # But fetching counter changes counter!
Number of instances created:  4

Unfortunately, as mentioned earlier, such an approach is completely unworkable if we don’t have an instance available, and making an instance changes the class data, as illustrated in the last line here. A better solution would be to somehow mark a method inside a class as never requiring an instance. The next section shows how.