Applying Any Decorator to Methods

The prior metaclass example works for just one specific function decorator—tracing. However, it’s trivial to generalize this to apply any decorator to all the methods of a class. All we have to do is add an outer scope layer to retain the desired decorator, much like we did for decorators in the prior chapter. The following, for example, codes such a generalization and then uses it to apply the tracer decorator again:

# Metaclass factory: apply any decorator to all methods of a class

from types import FunctionType
from mytools import tracer, timer

def decorateAll(decorator):
    class MetaDecorate(type):
        def __new__(meta, classname, supers, classdict):
            for attr, attrval in classdict.items():
                if type(attrval) is FunctionType:
                    classdict[attr] = decorator(attrval)
            return type.__new__(meta, classname, supers, classdict)
    return MetaDecorate

class Person(metaclass=decorateAll(tracer)):       # Apply a decorator to all
    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())

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When this code is run as it is, the output is again the same as that of the previous examples—we’re still ultimately decorating every method in a client class with the tracer function decorator, but we’re doing so in a more generic fashion:

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

Now, to apply a different decorator to the methods, we can simply replace the decorator name in the class header line. To use the timer function decorator shown earlier, for example, we could use either of the last two header lines in the following when defining our class—the first accepts the timer’s default arguments, and the second specifies label text:

class Person(metaclass=decorateAll(tracer)):               # Apply tracer

class Person(metaclass=decorateAll(timer())):              # Apply timer, defaults

class Person(metaclass=decorateAll(timer(label='**'))):    # Decorator arguments

Notice that this scheme cannot support nondefault decorator arguments differing per method, but it can pass in decorator arguments that apply to all methods, as done here. To test, use the last of these metaclass declarations to apply the timer, and add the following lines at the end of the script:

# If using timer: total time per method

print('-'*40)
print('%.5f' % Person.__init__.alltime)
print('%.5f' % Person.giveRaise.alltime)
print('%.5f' % Person.lastName.alltime)

The new output is as follows—the metaclass wraps methods in timer decorators now, so we can tell how long each and every call takes, for every method of the class:

**__init__: 0.00001, 0.00001
**__init__: 0.00001, 0.00002
Bob Smith Sue Jones
**giveRaise: 0.00001, 0.00001
110000.0
**lastName: 0.00001, 0.00001
**lastName: 0.00001, 0.00002
Smith Jones
----------------------------------------
0.00002
0.00001
0.00002