> how is that partisan politics?

Nope. Premise is backwards. Why does the mayor belong in a story about the intricacies of printing road signs? Is the mayor a road sign enthusiast? Did he create the road-sign printing system? No? No relation at all, other than seeing an opportunity to inject himself into a story?

Yeah, maybe don't include it, then.

> showing up as a mascot for a random human interest story is the primary function of a mayor.

That's certainly what politicians try to do. You, as a journalist, are not required to take the bait. Sometimes it's just cool to have a nice little story about the people at the sign factory.