Python decorator as a staticmethod

问题

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?

回答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.

回答2:

Python classes are created at runtime, after evaluating the contents of the class declaration. The class is evaluated by assigned all declared variables and functions to a special dictionary and using that dictionary to call type.__new__ (see customizing class creation).

So,

class A(B):
    c = 1

is equivalent to:

A = type.__new__("A", (B,), {"c": 1})

When you annotate a method with @staticmethod, there is some special magic that happens AFTER the class is created with type.__new__. Inside class declaration scope, the @staticmethod function is just an instance of a staticmethod object, which you can't call. The decorator probably should just be declared above the class definition in the same module OR in a separate "decorate" module (depends on how many decorators you have). In general decorators should be declared outside of a class. One notable exception is the property class (see properties). In your case having the decorator inside a class declaration might make sense if you had something like a color class:

class Color(object):

    def ___init__(self, color):
        self.color = color

     def ensure_same_color(f):
         ...

black = Color("black")

class TFord(object):
    def __init__(self, color):
        self.color = color

    @black.ensure_same_color
    def get():
        return 'Here is your shiny new T-Ford'

回答3:

ensure_black is returning a _aux method that isn't decorated by @staticmethod

You can return a non-static method to a static_method

http://docs.python.org/library/functions.html#staticmethod

来源：https://stackoverflow.com/questions/6412146/python-decorator-as-a-staticmethod

标签

python

decorator

static-methods