Dynamic method creation with decorators in Python

Question

This is hard. i want to not only create methods dynamically but also want to associate decorators with them. This is what I tried

import inspect
import types

class Dynamo(object):
    pass

def call_me_dec(func):
    print 'I am here'
    return func

def add_dynamo(cls,i):
    # @call_me_dec
    def innerdynamo(self):
        print "in dynamo %d" % i
        return i

    innerdynamo.__doc__ = "docstring for dynamo%d" % i
    innerdynamo.__name__ = "dynamo%d" % i
    setattr(cls, innerdynamo.__name__, innerdynamo)

def add_decorators(cls):
    for name, fn in inspect.getmembers(cls):
        if isinstance(fn, types.UnboundMethodType):
            setattr(cls, name, call_me_dec(fn))

for i in range(2):
    add_dynamo(Dynamo, i)

add_decorators(Dynamo)

d=Dynamo()
d.dynamo0()
d.dynamo1()

The output is:

    I am here
    I am here
    in dynamo 0
    in dynamo 1

Expected output:

    I am here
    in dynamo 0
    I am here
    in dynamo 1

Please explain why is this happening and how can I get the desired result?

Answer 1

You are seeing that because the decorator code is called at creation of the decorated function, not when the function is called. If you want to have the code run on call, you need to have the decorator return a callable (typically a closure) that calls the decorated method. Ex:

def call_me_dec(func):
    print 'Decorating %s' % func
    def func_wrapper(*args, **kwargs):
        print 'Calling %s' % func
        return func(*args, **kwargs)
    return func_wrapper

Decorator syntax is just syntactical sugar. Remember:

def func():
    pass
func = deco(func)

is equivalent to

@deco
def func():
    pass

Either way, you end up with the func label in the local namespace referencing the object that is returned from passing func as explicitly defined to deco. Decorator syntax bundles some extra checking (like making sure the input and output to and from deco are always callables), but basically, if something would work a given way in example 1, it will work that way in example 2.