Static Methods in 2.6 and 3.0

The concept of static methods is the same in both Python 2.6 and 3.0, but its implementation requirements have evolved somewhat in Python 3.0. Since this book covers both versions, I need to explain the differences in the two underlying models before we get to the code.

Really, we already began this story in the preceding chapter, when we explored the notion of unbound methods. Recall that both Python 2.6 and 3.0 always pass an instance to a method that is called through an instance. However, Python 3.0 treats methods fetched directly from a class differently than 2.6:

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  • In Python 2.6, fetching a method from a class produces an unbound method, which cannot be called without manually passing an instance.
  • In Python 3.0, fetching a method from a class produces a simple function, which can be called normally with no instance present.

In other words, Python 2.6 class methods always require an instance to be passed in, whether they are called through an instance or a class. By contrast, in Python 3.0 we are required to pass an instance to a method only if the method expects one—methods without a self instance argument can be called through the class without passing an instance. That is, 3.0 allows simple functions in a class, as long as they do not expect and are not passed an instance argument. The net effect is that:

 

 
  • In Python 2.6, we must always declare a method as static in order to call it without an instance, whether it is called through a class or an instance.
  • In Python 3.0, we need not declare such methods as static if they will be called through a class only, but we must do so in order to call them through an instance.

To illustrate, suppose we want to use class attributes to count how many instances are generated from a class. The following file, spam.py, makes a first attempt—its class has a counter stored as a class attribute, a constructor that bumps up the counter by one each time a new instance is created, and a method that displays the counter’s value. Remember, class attributes are shared by all instances. Therefore, storing the counter in the class object itself ensures that it effectively spans all instances:

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

The printNumInstances method is designed to process class data, not instance data—it’s about all the instances, not any one in particular. Because of that, we want to be able to call it without having to pass an instance. Indeed, we don’t want to make an instance to fetch the number of instances, because this would change the number of instances we’re trying to fetch! In other words, we want a self-less “static” method.

Whether this code works or not, though, depends on which Python you use, and which way you call the method—through the class or through an instance. In 2.6 (and 2.X in general), calls to a self-less method function through both the class and instances fail (I’ve omitted some error text here for space):

C:\misc> c:\python26\python
>>> from spam import Spam
>>> a = Spam()                   # Cannot call unbound class methods in 2.6
>>> b = Spam()                   # Methods expect a self object by default
>>> c = Spam()

>>> Spam.printNumInstances()
TypeError: unbound method printNumInstances() must be called with Spam instance
as first argument (got nothing instead)
>>> a.printNumInstances()
TypeError: printNumInstances() takes no arguments (1 given)

The problem here is that unbound instance methods aren’t exactly the same as simple functions in 2.6. Even though there are no arguments in the def header, the method still expects an instance to be passed in when it’s called, because the function is associated with a class. In Python 3.0 (and later 3.X releases), calls to self-less methods made through classes work, but calls from instances fail:

C:\misc> c:\python30\python
>>> from spam import Spam
>>> a = Spam()                       # Can call functions in class in 3.0
>>> b = Spam()                       # Calls through instances still pass a self
>>> c = Spam()

>>> Spam.printNumInstances()         # Differs in 3.0
Number of instances created:  3
>>> a.printNumInstances()
TypeError: printNumInstances() takes no arguments (1 given)

That is, calls to instance-less methods like printNumInstances made through the class fail in Python 2.6 but work in Python 3.0. On the other hand, calls made through an instance fail in both Pythons, because an instance is automatically passed to a method that does not have an argument to receive it:

Spam.printNumInstances()             # Fails in 2.6, works in 3.0
instance.printNumInstances()         # Fails in both 2.6 and 3.0

If you’re able to use 3.0 and stick with calling self-less methods through classes only, you already have a static method feature. However, to allow self-less methods to be called through classes in 2.6 and through instances in both 2.6 and 3.0, you need to either adopt other designs or be able to somehow mark such methods as special. Let’s look at both options in turn.