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?
Like __next__ method is looked up in the class, and for the class X that
would be its metaclass, type. So you have to adjust that:
$ cat contains.py
class XType(type):
def __contains__(cls, v):
print("metaclass")
return v in cls._ALL
class X(metaclass=XType):
ONE = "one"
TWO = "two"
_ALL = frozenset(v for k, v in locals().items() if k.isupper())
@classmethod
def __contains__(cls, v):
print("instance")
return v in cls
print("one" in X, "one" in X())
print("three" in X, "three" in X())
$ python3 contains.py
metaclass
instance
metaclass
True True
metaclass
instance
metaclass
False False
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