__contains__ classmethod?

[Tim Chase]

Playing around, I had this (happens to be Py2, but gets the same
result in Py3) code

class X(object):
  ONE = "one"
  TWO = "two"
  _ALL = frozenset(v for k,v in locals().items() if k.isupper())
  @classmethod
  def __contains__(cls, v):
    return v in cls._ALL
print(dir(X))
print(X._ALL)

Running this gives

  ['ONE', 'TWO', '_ALL', '__class__', '__contains__', '__delattr__',
  '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__',
  '__getattribute__', '__gt__', '__hash__', '__init__', '__le__',
  '__lt__', '__module__', '__ne__', '__new__', '__reduce__',
  '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__',
  '__str__', '__subclasshook__', '__weakref__']

And then, depending on whether it's Py2 or Py3, I get either

  frozenset({'one', 'two'})
  frozenset(['two', 'one'])

Which I expect.  Hey, look. There's a __contains__ method. And it
has been specified as a @classmethod.  So I want to test it:

  print("one" in X)

However that fails with

  Traceback (most recent call last):
    File "x.py", line 10, in <module>
      print("one" in X)
  TypeError: argument of type 'type' is not iterable

My understanding was that "in" makes use of an available __contains__
but something seems to preventing Python from finding that.

What's going on here?

Thanks,

-tkc