Hi! Thanks for making this library! :)
I recently had a bug that boiled down to openpyxl saying "Cannot convert <random string> to Excel". It turned out that my project was passing a Django object to openpyxl and the error message contained its
__str__. Could you either use
__repr__ in the error message so it's easier to debug or - preferably - try calling
__str__ on an unknown object instead of crashing?