Calling class staticmethod within the class body

Question

When I attempt to use a static method from within the body of the class  and define the static method using the built-in staticmethod function as a decorator  like this   class Klass object         staticmethod    use as decorator     def  stat func            return 42       ANS    stat func      call the staticmethod      def method self           ret   Klass  stat func     Klass  ANS         return ret   I get the following error   Traceback  most recent call last   lt br gt    File  call staticmethod py   line 1  in  lt module gt      class Klass object      File  call staticmethod py   line 7  in Klass      ANS    stat func      TypeError   staticmethod  object is not callable   I understand why this is happening  descriptor binding   and can work around it by manually converting  stat func   into a staticmethod after its last use  like so   class Klass object        def  stat func            return 42       ANS    stat func      use the non-staticmethod version       stat func   staticmethod  stat func     convert function to a static method      def method self           ret   Klass  stat func     Klass  ANS         return ret   So my question is   Are there better  as in cleaner or more  Pythonic   ways to accomplish this

User · Accepted Answer

staticmethod objects apparently have a   func   attribute storing the original raw function  makes sense that they had to   So this will work   class Klass object         staticmethod    use as decorator     def stat func            return 42       ANS   stat func   func        call the staticmethod      def method self           ret   Klass stat func           return ret     As an aside  though I suspected that a staticmethod object had some sort of attribute storing the original function  I had no idea of the specifics  In the spirit of teaching someone to fish rather than giving them a fish  this is what I did to investigate and find that out  a C amp P from my Python session     gt  gt  gt  class Foo object            staticmethod         def foo                return 3         global z         z   foo   gt  gt  gt  z  lt staticmethod object at 0x0000000002E40558 gt   gt  gt  gt  Foo foo  lt function foo at 0x0000000002E3CBA8 gt   gt  gt  gt  dir z      class        delattr        doc        format        func        get        getattribute        hash        init        new        reduce        reduce ex        repr        setattr        sizeof        str        subclasshook      gt  gt  gt  z   func    lt function foo at 0x0000000002E3CBA8 gt    Similar sorts of digging in an interactive session  dir is very helpful  can often solve these sorts of question very quickly

User · Answer

This is due to staticmethod being a descriptor and requires a class-level attribute fetch to exercise the descriptor protocol and get the true callable   From the source code      It can be called either on the class  e g  C f    or on an instance    e g  C   f     the instance is ignored except for its class    But not directly from inside the class while it is being defined    But as one commenter mentioned  this is not really a  Pythonic  design at all  Just use a module level function instead

User · Answer

What about injecting the class attribute after the class definition   class Klass object         staticmethod    use as decorator     def stat func            return 42      def method self           ret   Klass stat func           return ret  Klass  ANS   Klass stat func      inject the class attribute with static method value

User · Answer

What about this solution  It does not rely on knowledge of  staticmethod decorator implementation  Inner class StaticMethod plays as a container of static initialization functions   class Klass object        class StaticMethod           staticmethod    use as decorator         def  stat func                return 42       ANS   StaticMethod  stat func      call the staticmethod      def method self           ret   self StaticMethod  stat func     Klass  ANS         return ret

User · Answer

This is the way I prefer   class Klass object         staticmethod     def stat func            return 42       ANS   stat func   func          def method self           return self   class   stat func     self   class    ANS   I prefer this solution to Klass stat func  because of the DRY principle  Reminds me of the reason why there is a new super   in Python 3     But I agree with the others  usually the best choice is to define a module level function   For instance with  staticmethod function  the recursion might not look very good  You would need to break DRY principle by calling Klass stat func inside Klass stat func   That s because you don t have reference to self inside static method  With module level function  everything will look OK

User · Answer

If the  quot core problem quot  is assigning class variables using functions  an alternative is to use a metaclass  it s kind of  quot annoying quot  and  quot magical quot  and I agree that the static method should be callable inside the class  but unfortunately it isn t   This way  we can refactor the behavior into a standalone function and don t clutter the class  class KlassMetaClass type object         staticmethod     def  stat func            return 42      def   new   cls  clsname  bases  attrs             Call the   new   method from the Object metaclass         super new   super     new   cls  clsname  bases  attrs            Modify class variable  quot  ANS quot          super new  ANS   cls  stat func           return super new  class Klass object  metaclass KlassMetaClass        quot  quot  quot      Class that will have class variables set pseudo-dynamically by the metaclass      quot  quot  quot      pass  print Klass  ANS    prints 42  Using this alternative  quot in the real world quot  may be problematic  I had to use it to override class variables in Django classes  but in other circumstances maybe it s better to go with one of the alternatives from the other answers

[python] Calling class staticmethod within the class body?

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