Input inference in Python boxes is too aggressive

[darabos]

Specifically, if you have an output, it can be picked up as a non-existent input. E.g.

• Inputs: <infer from code>
• Outputs: vs.x: float
• Code: vs['x'] = vs['name'].str.len()

This will tell you that vs.x is not a valid input.

I'm not sure what's the best solution. Maybe just drop missing inputs in "infer" mode. They may be an output, something in a comment, etc. We will get an error anyway when we try to use them.

(If someone hits this, the workaround is to use "infer" for both inputs and output, or for neither of them.)