__contains__ classmethod?

[MRAB]

On 2017-12-18 21:25, Tim Chase wrote:
> 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?
> 
Maybe it's just because it's intended to be an instance method, like 
__getitem__.