Code below is tested with 2.5, 2.7 and 3.3

fixed.py

from functools import wraps
import inspect

# Fixed code

def initializer(fun):
    names, varargs, keywords, defaults = inspect.getargspec(fun)
    @wraps(fun)
    def wrapper(self, *args, **kargs):
        for name, arg in zip(names[1:], args) + kargs.items():
            setattr(self, name, arg)
        for i in range(len(defaults)):
            index = -(i + 1)
            if not hasattr(self, names[index]):
                setattr(self, names[index], defaults[index])
        fun(self, *args, **kargs)
    return wrapper


class Foo(object):
    @initializer
    def __init__(self, a, b, c=None, d=None, e=3):
        pass
    
f = Foo(1, 2, d="a")
assert f.a == 1
assert f.b == 2
assert f.c is None
assert f.d == "a"
assert f.e == 3

print("Ok")

fixed3.py

from functools import wraps
import inspect

# Python3 compatible code (it runs fine under python2 too).
def initializer(fun):
    names, varargs, keywords, defaults = inspect.getargspec(fun)
    @wraps(fun)
    def wrapper(self, *args, **kargs):
        for name, arg in list(zip(names[1:], args)) + list(kargs.items()):
            setattr(self, name, arg)
        for i in range(len(defaults)):
            index = -(i + 1)
            if not hasattr(self, names[index]):
                setattr(self, names[index], defaults[index])
        fun(self, *args, **kargs)
    return wrapper


class Foo(object):
    @initializer
    def __init__(self, a, b, c=None, d=None, e=3):
        pass
    
f = Foo(1, 2, d="a")
assert f.a == 1
assert f.b == 2
assert f.c is None
assert f.d == "a"
assert f.e == 3

print("Ok")

original.py

from functools import wraps
import inspect

# Original code

def initializer(fun):
    names, varargs, keywords, defaults = inspect.getargspec(fun)
    @wraps(fun)
    def wrapper(self, *args, **kargs):
        for name, arg in zip(names[1:], args) + kargs.items():
            setattr(self, name, arg)
        for i in range(len(defaults)):
            if not hasattr(self, names[-(i+1)]):
                setattr(self, names[-(i+1)], defaults[i])    # defaults[i] should be defaults[-(i + 1)]
        fun(self, *args, **kargs)
    return wrapper

class Foo(object):
    @initializer
    def __init__(self, a, b, c=None, d=None, e=3):
        pass
    
f = Foo(1, 2, d="a")
assert f.a == 1
assert f.b == 2
assert f.c is None    # This fails (f.c == 3)
assert f.d == "a"
assert f.e == 3       # This fails (f.e is None)

print("Ok")