Edit: It seems I misread your post

Hi, I always use a PythonCaller to expose attributes stored in a User Parameter:

This can also be done in a FeatureReader

Hope this helps!

Thank you for your response. I have already tried setting the user parameter for the Attribute to expose. During execution, I set $(OUTFIELDS) to "OBJECTID MILLESIME_DATA MILLESIME_DATA2", and I get all three fields in my output. However, when I change the value of $(OUTFIELDS) to "OBJECTID MILLESIME_DATA", I still see all three fields instead of just the two specified. Therefore, I'm looking for an alternative solution, if possible.

I think the way to do this is to use dynamic writing (define the attributes based on a schema feature).

One way to do this is to use the SchemaScanner to generate a SchemaFeature. You will then need to remove the attributes from the schema feature which are not in the outfields parameter.

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Attached sample demonstrating this.

Interesting, I have never seen this issue. It seems like the fields to be exposed in the PythonCaller are saved in memory or something. 
I always feed the field names into the User Parameter through a WorkspaceRunner. If the workspace is opened again (by rerunning the WorkspaceRunner), I guess the saved field names are reset.

@nielsgerrits Thank you for your example. I’ll try it and keep you informed.

@joepk It's very strange because when I reopen the application, it recognizes the new value of the user parameter. I tried publishing my workspace to FME Flow, but I still have the same issue.