Nitpicky, but maybe this should return and the assert should be where it's called? That way an error would show the line with the exception variable instead of just the parameters.

I based it on the round_trip_test() from json_snippet.py where it has similar logic. But I agree it’d be a bit nicer to move asserts out of the function. Shall I change it just here, or in both snippets (I haven’t found any other round-tripy utils)?