Hans Palacios

Good point. I haven't yet found a comprehensive list in the docs about each and every available tag, with the exception of the ones listed under Built-in Tags.
Reference: https://www.sidefx.com/docs/houdini/ref/windows/optype.html#tags [www.sidefx.com]

If the tags()method returns an empty dictionary, it's likely that it's indicating that no tags have yet been set. You can use setTags()to specify a custom tag or something like setScriptCallbackLanguage(), which would then list script_callback_language as one of the tag keys.

Your simplified test case should work if the f@attr1 = 5.0is first applied to each vertex in its own Vertex Wrangle and then the next Vertex Wrangle can read the attribute using the 2nd and 3rd lines of code just as you've written them.