Function Decorator Basics

Syntactically, a function decorator is a sort of runtime declaration about the function that follows. A function decorator is coded on a line by itself just before the def statement that defines a function or method. It consists of the @ symbol, followed by what we call a metafunction—a function (or other callable object) that manages another function. Static methods today, for example, may be coded with decorator syntax like this:

class C:
   @staticmethod                                 # Decoration syntax
   def meth():
       ...

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Internally, this syntax has the same effect as the following (passing the function through the decorator and assigning the result back to the original name):

class C:
   def meth():
       ...
   meth = staticmethod(meth)                     # Rebind name

Decoration rebinds the method name to the decorator’s result. The net effect is that calling the method function’s name later actually triggers the result of its staticmethod decorator first. Because a decorator can return any sort of object, this allows the decorator to insert a layer of logic to be run on every call. The decorator function is free to return either the original function itself, or a new object that saves the original function passed to the decorator to be invoked indirectly after the extra logic layer runs.

With this addition, here’s a better way to code our static method example from the prior section in either Python 2.6 or 3.0 (the classmethod decorator is used the same way):

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

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

a = Spam()
b = Spam()
c = Spam()
Spam.printNumInstances()      # Calls from both classes and instances work now!
a.printNumInstances()         # Both print "Number of instances created:  3"

Keep in mind that staticmethod is still a built-in function; it may be used in decoration syntax, just because it takes a function as argument and returns a callable. In fact, any such function can be used in this way—even user-defined functions we code ourselves, as the next section explains.