How to avoid "()" when writing a decorator accepting optional arguments?

[Giampaolo Rodolà]

I've written this decorator to deprecate a function and (optionally)
provide a callable as replacement

     def deprecated(repfun=None):
        """A decorator which can be used to mark functions as deprecated.
        Optional repfun is a callable that will be called with the same args
        as the decorated function.
        """
        def outer(fun):
            def inner(*args, **kwargs):
                msg = "%s is deprecated" % fun.__name__
                if repfun is not None:
                    msg += "; use %s instead" % (repfun.__name__)
                warnings.warn(msg, category=DeprecationWarning, stacklevel=2)
                if repfun is not None:
                    return repfun(*args, **kwargs)
                else:
                    return fun(*args, **kwargs)
            return inner
        return outer


Now, I can use my decorator as such:

    @deprecated()
    def foo():
        return 0

...or provide an optional argument:

    @deprecated(some_function)
    def foo():
        return 0

...but I don't know how to modify it so that I can omit parentheses:

    @deprecated
    def foo():
        return 0

Any hint?


--- Giampaolo
http://code.google.com/p/pyftpdlib/
http://code.google.com/p/psutil/