Deriving output filename from another parameter in Python Script Tool?

Question

In an ArcGIS Toolbox script tool, how can I get the default value for one parameter to be derived from the value of another parameter?

GPX To Features does this, but I haven't been able to figure out how. There is logic under Tool >> Properties >> Validation to sanitize the filename but nothing about setting a default value. I've copied the validation code verbatim into my tool but that doesn't work.

Answer 1

This has had me spinning my wheels on and off for years. I didn't see what was wrong because I was always looking a layer deeper, into the code of the validation class and in the script.

The solution is simple: Remember to set the direction of the output parameter!