I have some code that calculates a value that changes per time sample and is commonly used by artists to drive parameters on their nodes (think something akin to $FF). While storing the function on hou.session and then referencing it in the parameters is an option, I'd much prefer to provide an environment variable or something else equally convenient to make life easier for the artists.

Does anyone know how to drive an environment variable via Python and have it update per time sample on any parameters that reference it? Also, what is the best way to trigger the function to update the variable once every time sample?

You can place code inside the frame change event, if that's what you mean by time sample?

Try adding a callback.
https://www.sidefx.com/docs/houdini/hom/hou/playbar.html [www.sidefx.com]

That does sound promising. So I can update the environment variable there, but how would I then update the parameters that are using that variable. I tried hou.hscript("varchange") as recommended @ https://www.sidefx.com/forum/topic/58619/, [www.sidefx.com] but that didn't seem to work: at least not in LOPs. Any idea why that would be?

Edit: I just realized that the update issue might be because I was trying to trigger the function from a parameter, so it might be possible it was signaling the varchange after the parameters using the variable had already been updated. I'll test and report back.

So using the playbar callback https://www.sidefx.com/docs/houdini/hom/hou/playbar.html [www.sidefx.com] seemed to work perfectly at first (my issue was indeed due to the fact that varchange doesn't work properly when called from a parameter). However I've discovered that ROPs do not trigger the callback, and so the environment variable does not update during caching or rendering.

Any ideas if there is a callback I can use that is run both by the viewport and by background processes like ROPs?