`inspect.getargspec` works only for functions and raises an exception for callable classes. Though, you could call `getargspec` on `obj.__call__` (which will return also the `self` argument).

+1 for a proper fix. Moments ago I was wondering about this error message because my function was throwing a TypeError:
```The function 'fn' provided to 'Node: Node' takes 1 argument(s), where a function for this type of node is expected to take 1 argument(s)```