Tracing with Metaclasses and Decorators

The manual decoration scheme of the prior section works, but it requires us to add decoration syntax before each method we wish to trace and to later remove that syntax when we no longer desire tracing. If we want to trace every method of a class, this can become tedious in larger programs. It would be better if we could somehow apply the tracer decorator to all of a class’s methods automatically.

With metaclasses, we can do exactly that—because they are run when a class is constructed, they are a natural place to add decoration wrappers to a class’s methods. By scanning the class’s attribute dictionary and testing for function objects there, we can automatically run methods through the decorator and rebind the original names to the results. The effect is the same as the automatic method name rebinding of decorators, but we can apply it more globally:

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# Metaclass that adds tracing decorator to every method of a client class

from types import FunctionType
from mytools import tracer

class MetaTrace(type):
    def __new__(meta, classname, supers, classdict):
        for attr, attrval in classdict.items():
            if type(attrval) is FunctionType:                      # Method?
                classdict[attr] = tracer(attrval)                  # Decorate it
        return type.__new__(meta, classname, supers, classdict)    # Make class

class Person(metaclass=MetaTrace):
    def __init__(self, name, pay):
        self.name = name
        self.pay  = pay
    def giveRaise(self, percent):
        self.pay *= (1.0 + percent)
    def lastName(self):
        return self.name.split()[-1]

bob = Person('Bob Smith', 50000)
sue = Person('Sue Jones', 100000)
print(bob.name, sue.name)
sue.giveRaise(.10)
print(sue.pay)
print(bob.lastName(), sue.lastName())

When this code is run, the results are the same as before—calls to methods are routed to the tracing decorator first for tracing, and then propagated on to the original method:

call 1 to __init__
call 2 to __init__
Bob Smith Sue Jones
call 1 to giveRaise
110000.0
call 1 to lastName
call 2 to lastName
Smith Jones

The result you see here is a combination of decorator and metaclass work—the metaclass automatically applies the function decorator to every method at class creation time, and the function decorator automatically intercepts method calls in order to print the trace messages in this output. The combination “just works,” thanks to the generality of both tools.