This looks correct. I wonder if bare backslashes in values (Windows paths, regex patterns, etc.) could still break things, since the template doesn't escape them either. Would parts[idx] = json.dumps(parts[idx])[1:-1] on the odd parts work as an alternative?

Yes, that is much simpler and seems to work better as well. I was looking for this specific functionality, but didn't think to dump a string back to a string, thus escaping it. That's nice!