Hi,
the exception message must be longer as just the one line you posted . It would be helpful to know this error message.
However, I think it complains about the missing variables definition what you already have assumed.
The variables are not directly listed in the parameters list when you call gpt -h Mosaic. The reason is that it is are more complex parameter and it cannot be specified on the command line. It can only be specified within a xml file or via the the API. The variables and the conditions are shown in the example xml on the command line when you type gpt -h Mosaic.
You can use it in your code as follows:
variables = MosaicOp.Variable('name', 'expression'), MosaicOp.Variable('name2', 'expression2')
parameters.put('variables', variables)
Don’t take the syntax to serious. I’m not a python expert.