@staticmethod with @property

Question

I want

Stats.singleton.twitter_count += 1 

and I thought I could do

class Stats:     singleton_object = None      @property     @staticmethod     def singleton():         if Stats.singleton_object:             return Stats.singleton_object         Stats.singleton_object = Stats()         return Stats.singleton() 

But it throws an exception:

>>> Stats.singleton.a = "b" Traceback (most recent call last):   File "<stdin>", line 1, in <module> TypeError: 'property' object has only read-only attributes (assign to .a) 

Answer 1

User kaizer.se was onto something as far as the original question goes. I took it a step further in terms of simplicity, so that it now requires only a single decorator:

class classproperty(property):     def __get__(self, cls, owner):         return classmethod(self.fget).__get__(None, owner)() 

Usage:

class Stats:     _current_instance = None      @classproperty     def singleton(cls):         if cls._current_instance is None:             cls._current_instance = Stats()         return cls._current_instance 

As noted, this way of creating a singleton is not a good design pattern; if that must be done, a metaclass factory is a much better way to do it. I was just excited about the prospect of a class property though, so, there it is.

Answer 2

I guess giving a Python code snippet to show how do property and staticmethod work would be helpful.

Both of them are descriptors which implements __get__ or __set__

property is a data descriptor

class Property(object):     "Emulate PyProperty_Type() in Objects/descrobject.c"      def __init__(self, fget=None, fset=None, fdel=None, doc=None):         self.fget = fget         self.fset = fset         self.fdel = fdel         if doc is None and fget is not None:             doc = fget.__doc__         self.__doc__ = doc      def __get__(self, obj, objtype=None):         if obj is None:             return self         if self.fget is None:             raise AttributeError("unreadable attribute")         return self.fget(obj)      def __set__(self, obj, value):         if self.fset is None:             raise AttributeError("can't set attribute")         self.fset(obj, value)      def __delete__(self, obj):         if self.fdel is None:             raise AttributeError("can't delete attribute")         self.fdel(obj)      def getter(self, fget):         return type(self)(fget, self.fset, self.fdel, self.__doc__)      def setter(self, fset):         return type(self)(self.fget, fset, self.fdel, self.__doc__)      def deleter(self, fdel):         return type(self)(self.fget, self.fset, fdel, self.__doc__) 

And staticmethod is a non-data descriptor

class StaticMethod(object):     "Emulate PyStaticMethod_Type() in Objects/funcobject.c"      def __init__(self, f):         self.f = f      def __get__(self, obj, objtype=None):         return self.f