Question

I am trying to write a decorator that gets a single arg, i.e

@Printer(1)
def f():
    print 3

So, naively, I tried:

class Printer:
    def __init__(self,num):
         self.__num=num
    def __call__(self,func):
         def wrapped(*args,**kargs):
              print self.__num
              return func(*args,**kargs**)
         return wrapped

This is ok, but it also works as a decorator receiving no args, i.e

@Printer
def a():
   print 3

How can I prevent that?

Answer 1

Well, it’s already effectively prevented, in the sense that calling a() doesn’t work.

But to stop it as the function is defined, I suppose you’d have to change __init__ to check the type of num:

def __init__(self,num):
    if callable(num):
        raise TypeError('Printer decorator takes an argument')
    self.__num=num

I don’t know if this is really worth the bother, though. It already doesn’t work as-is; you’re really asking to enforce the types of arguments in a duck-typed language.