Likely known by many here, however, I can’t see it documented officially so I’m going to spell it out.

If you have created a CustomTranformer or are working with one you need to be very careful how you use/reference parameters.

There are many gotchas when working with CustomTransformers but this is the one that trips me up the most. 

If you define a parameter in a Custom Transform with an attribute value this will not always be treated as a Global Parameter inside the transformer. I.e., depending on what happens in the Transformer the value will not always be accessible.

I saw a great comment by ​@ctredinnick in this thread from a couple of years ago: 

where they mention that Parameter Values defined by an attribute value will be treated in a similar way to unexposed attributes.

I had always assumed that Parameter values were somehow ‘locked in’ of defined for every feature when they went in but that’s not true. Anytime the parameter is referenced inside a CustomTransformer it will just resolve the @Value(<attribute-name>). 

It actually just seems so obvious now that I’m explaining it but often when you’re initially building a CustomTransfomer it’s not an issue - for example you could have a CustomTransformer which includes a FeatureReader or an SQLExcecutor follow by a tester to filter features further based on a parameter value. Unless you make sure that you merge attributes this wont work if you’ve defined the parameter with an attribute value.

I’ve come to the conclusion that if I’m building a CustomTransformer it’s almost the safest option to just always use a ParameterFetcher right at the start to fetch them all - but this is obviously annoying because it’s really nice to just be able to reference an parameter value, this is also nice because you can easily see where the value is referenced.

Anyone feel my pain on this one?