Shortly after semi-finishing pyparsing_helper, I realized that it's really not needed, because there's a tool called Reinteract that can fill the same role, plus much more.
Reinteract is a graphical Python session that lets you tinker with your code at any point and get an immediate recalculation. That's what pyparsing_helper does, too, but pyparsing_helper is specialized for pyparsing use, whereas reinteract is suitable for any Python. Here's reinteract applied to a pyparsing example.
Reinteract is also more sophisticated and versatile, so I'm afraid pyparsing_helper's short time in the sun has been eclipsed. I'm happy to have found this new tool, though!