Use __class__ when comparing meta data

[madsbk]

In order to support object wrappers/proxies like rapidsai/dask-cuda#451 and weakref.proxy, this PR implements type checking through x.__class__ instead of type(x).

• Tests added / passed
• Passes black dask / flake8 dask

[jacobtomlinson]

I am a little out of my depth here, so can I just ask what are the differences between this and other options:

• if not isinstance(x, type(meta)):
• if type(x) != type(meta):
• if x.__class__ != meta.__class__:

[martindurant]

I guess type might return something like ProxyType, but .__class__ would fetch the attribute value from the target object, and so give the underlying class.
isinstance seems to work, though - I suppose it uses .__class__ too?

[madsbk]

I am a little out of my depth here, so can I just ask what are the differences between this and other options:

• if not isinstance(x, type(meta)):
meta is allowed to be a subclass of x and isinstance(x) uses x.__class__ get type info.

• if type(x) != type(meta):
  The types must be the same and type(x) always returns the exact class of x

• if x.__class__ != meta.__class__:
  In most cases this is the same as using type(x) but can be overwritten, which is how Individual CUDA object spilling  rapidsai/dask-cuda#451 and weakref.proxy works.

[madsbk]

I guess type might return something like ProxyType, but .__class__ would fetch the attribute value from the target object, and so give the underlying class.

Exactly, isinstance(x) should work just as well with proxy objects but in order to catch mismatch between derived types, I think x.__class__ make more sense.

[jacobtomlinson]

Thanks for unpacking that here. Seems like a reasonable change. I think it's good to have this written down here in case anyone needs to refer back to it.

Just to check my understanding does that mean if not isinstance(x, meta.__class__): would also be valid?

[madsbk]

Just to check my understanding does that mean if not isinstance(x, meta.__class__): would also be valid?

Not exactly, in the case of isinstance(x, meta.__class__), x is allowed to be a subclass of meta whereas in x.__class__ != meta.__class__ both must be the same class (or proxy thereof).

[jacobtomlinson]

Ok thanks!

Also thanks for the comments, I think that is really helpful.