Returns the number of explicit non-keyword arguments that the callable
    can be called with.

    Bound methods are called with an implicit first argument, so this takes
    that into account.

    Excludes *args and **kwargs declarations.

def fn_available_argcount(callable):
    """Returns the number of explicit non-keyword arguments that the callable
    can be called with.

    Bound methods are called with an implicit first argument, so this takes
    that into account.

    Excludes *args and **kwargs declarations.
    """
    fn = get_fn_or_method(callable)
    if _inspect.isfunction(fn):
        return fn.__code__.co_argcount
    else: # method
        if fn.__self__ is None:
            return fn.__func__.__code__.co_argcount
        else:
            return fn.__func__.__code__.co_argcount - 1