Use 'S' format character for the optional constructor argument, so we get a decent error message when it's not a string (instead of confusing errors when trying to use the thing).
Use 'S' format character for the optional constructor argument, so we
get a decent error message when it's not a string (instead of
confusing errors when trying to use the thing).

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator