Python decorator as a staticmethod

Question

I'm trying to write a python class which uses a decorator function that needs information of the instance state. This is working as intended, but if I explicitly make the decorator a staticmetod, I get the following error:

Traceback (most recent call last):   File "tford.py", line 1, in <module>     class TFord(object):   File "tford.py", line 14, in TFord     @ensure_black TypeError: 'staticmethod' object is not callable 

Why?

Here is the code:

class TFord(object):     def __init__(self, color):         self.color = color      @staticmethod     def ensure_black(func):         def _aux(self, *args, **kwargs):             if self.color == 'black':                 return func(*args, **kwargs)             else:                 return None         return _aux      @ensure_black     def get():         return 'Here is your shiny new T-Ford'  if __name__ == '__main__':     ford_red = TFord('red')     ford_black = TFord('black')      print ford_red.get()     print ford_black.get() 

And if I just remove the line @staticmethod, everything works, but I do not understand why. Shouldn't it need self as a first argument?

Answer 1

This is not how staticmethod is supposed to be used. staticmethod objects are descriptors that return the wrapped object, so they only work when accessed as classname.staticmethodname. Example

class A(object):     @staticmethod     def f():         pass print A.f print A.__dict__["f"] 

prints

<function f at 0x8af45dc> <staticmethod object at 0x8aa6a94> 

Inside the scope of A, you would always get the latter object, which is not callable.

I'd strongly recommend to move the decorator to the module scope -- it does not seem to belong inside the class. If you want to keep it inside the class, don't make it a staticmethod, but rather simply del it at the end of the class body -- it's not meant to be used from outside the class in this case.