`staticmethod` and `abc.abstractmethod`: Will it blend?

Question

In my Python app I want to make a method that is both a staticmethod and an abc.abstractmethod. How do I do this?

I tried applying both decorators, but it doesn't work. If I do this:

import abc

class C(object):
    __metaclass__ = abc.ABCMeta

    @abc.abstractmethod
    @staticmethod    
    def my_function(): pass

I get an exception*, and if I do this:

class C(object):
    __metaclass__ = abc.ABCMeta

    @staticmethod    
    @abc.abstractmethod
    def my_function(): pass

The abstract method is not enforced.

How can I make an abstract static method?

*The exception:

File "c:\Python26\Lib\abc.py", line 29, in abstractmethod
 funcobj.__isabstractmethod__ = True
AttributeError: 'staticmethod' object has no attribute '__isabstractmethod__'

Answer 1

class abstractstatic(staticmethod):
    __slots__ = ()
    def __init__(self, function):
        super(abstractstatic, self).__init__(function)
        function.__isabstractmethod__ = True
    __isabstractmethod__ = True

class A(object):
    __metaclass__ = abc.ABCMeta
    @abstractstatic
    def test():
        print 5

Answer 2

Starting with Python 3.3, it is possible to combine @staticmethod and @abstractmethod, so none of the other suggestions are necessary anymore:

@staticmethod
@abstractmethod
def my_abstract_staticmethod(...):

Answer 3

This will do it:

  >>> import abc
  >>> abstractstaticmethod = abc.abstractmethod
  >>>
  >>> class A(object):
  ...     __metaclass__ = abc.ABCMeta
  ...     @abstractstaticmethod
  ...     def themethod():
  ...          pass
  ... 
  >>> a = A()
  >>> Traceback (most recent call last):
  File "asm.py", line 16, in <module>
    a = A()
  TypeError: Can't instantiate abstract class A with abstract methods test

You go "Eh? It just renames @abstractmethod", and this is completely correct. Because any subclass of the above will have to include the @staticmethod decorator anyway. You have no need of it here, except as documentation when reading the code. A subclass would have to look like this:

  >>> class B(A):
  ...     @staticmethod
  ...     def themethod():
  ...         print "Do whatevs"

To have a function that would enforce you to make this method a static method you would have to subclass ABCmeta to check for that and enforce it. That's a lot of work for no real return. (If somebody forgets the @staticmethod decorator they will get a clear error anyway, it just won't mention static methods.

So in fact this works just as well:

  >>> import abc
  >>>
  >>> class A(object):
  ...     __metaclass__ = abc.ABCMeta
  ...     @abc.abstractmethod
  ...     def themethod():
  ...         """Subclasses must implement this as a @staticmethod"""
  ...          pass

Update - Another way to explain it:

That a method is static controls how it is called. An abstract method is never called. And abstract static method is therefore a pretty pointless concept, except for documentation purposes.

Answer 4

This is currently not possible in Python 2.X, which will only enforce the method to be abstract or static, but not both.

In Python 3.2+, the new decoratorsabc.abstractclassmethod and abc.abstractstaticmethod were added to combine their enforcement of being abstract and static or abstract and a class method.

See Python Issue 5867