A circular reference problem was fixed in the Python SDKs. This should have only existed if your API description had a DateTime property in a model.
Please regenerate your Python SDK to get the bug fix.
The api_helper module in Python SDKs imported the base_model module and the api_helper module was, in some cases, imported in modules which imported the base_model module. This caused a circular reference which we have now fixed.
The Python interpreter reported this problem as something like:
ImportError: cannot import name <ModelName>