PythonCaller has no attributes to set apart from grouping and output parameters, and of course the code.
Any input parameters to be set outside the code, needs to be set in another transformer like AttributeCreator.
It would be nice to have a small list of input parameters, so one can easily change input parameters without changing the code. Just a simple name/value list, just like other transformers. The values could then in turn be tied to user parameters in the workspace/custom transformer if need be.