[Python-Dev] deprecation of abstractstaticmethod and abstractclassmethod

[Ethan Furman]

In issue 11610* abstractclassmethod and abstractstaticmethod were deprecated, apparently because they were redundant with the new technique of calling `classmethod` or `staticmethod` followed by a call to `abstractmethod`.  To put it in code:

# deprecated

class Foo(ABC):

     @abstractclassmethod
     def foo_happens(cls):
         # do some fooey stuff

# the new(er) way

class Foo(ABC):

     @classmethod
     @abstractmethod
     def foo_happens(cls):
         # do some fooey stuff


I would like to remove the deprecated status of `abstractclassmethod` and `abstractstaticmethod` mainly because:

- using the combined decorator is easy to get right
   (@abstractmethod followed by @classmethod doesn't work)

- getting the order wrong can be hard to spot and fix

Obviously, decorator order matters for many, if not most, decorators out there -- so why should these two be any different?  Because 'abstract', 'class', and 'static' are adjectives -- they're describing the method, rather than changing it**; to use an example, what is the difference between "hot, dry sand" and "dry, hot sand"?  The sand is just as dry and just as hot either way.  In a debugging session looking at:

    @abstractmethod
    @classmethod
    def some_func(self, this, that, the_other):
         # many
         # many
         ...
         ...
         ...
         # many
         # lines
         # of
         # code

Not noticing that the two decorators are in reverse order would be very easy to do.

Because order matters here, but cognitively should not, a helper function to make sure it is always done right is a prime candidate to be added to a module -- and, luckily for us, those helper functions already exist!  Unfortunately, they are also deprecated, discouraging their use, when we should be encouraging their use.

What are the reasons to /not/ remove the deprecation?

--
~Ethan~



* https://bugs.python.org/issue11610

** I realize that abstractmethod does actually change the function, but that's an implementation detail.