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For me the issue was resolved by following it on https://github.com/googleapis/python-ndb/issues/249
andrewsg commented 11 days ago:
I think we've identified an issue with devappserver related to the six library specifically. Could you please try a workaround? Add the line:
import six; reload(six)
to the top of your app, before NDB is loaded and let me know if that works.