Python 3.9 typing and multiple-inheritance

[Bleyddyn]

Trying to train using Python 3.9:

using donkey v4.3.5 ...
Traceback (most recent call last):
  File "/Users/Shared/Personal/mycar/./train.py", line 17, in <module>
    from donkeycar.pipeline.training import train
  File "/Users/Shared/Personal/donkeycar/donkeycar/pipeline/training.py", line 13, in <module>
    from donkeycar.pipeline.sequence import TubRecord, TubSequence, TfmIterator
  File "/Users/Shared/Personal/donkeycar/donkeycar/pipeline/sequence.py", line 45, in <module>
    class TfmIterator(Generic[R, XOut, YOut],  SizedIterator[Tuple[XOut, YOut]]):
  File "/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/abc.py", line 106, in __new__
    cls = super().__new__(mcls, name, bases, namespace, **kwargs)
TypeError: Cannot create a consistent method resolution
order (MRO) for bases Generic, SizedIterator

The solution seems to be to add intermediate classes. e.g.

-class TfmIterator(Generic[R, XOut, YOut],  SizedIterator[Tuple[XOut, YOut]]):
+class IntermediateIterator(Generic[R, XOut, YOut]):
+    def __init__(self) -> None:
+        pass
+
+class TfmIterator(IntermediateIterator[R, XOut, YOut],  SizedIterator[Tuple[XOut, YOut]]):

I haven't tested with older versions of Python, but I was able to get a training run to finish with 3.9.

I'm new to python typing and multiple inheritance, so I don't know if there's a better way to do this.

[DocGarbanzo]

There have been some impact changes in mypy between python 3.7 -> 3.9, which we have to address. Thanks for your comment on how to resolve them. We'll have a look once we'll work on that upgrade.