Read-only workbook is not serializable
A workbook being loaded with read_only=True cannot be serialized by pickler (pickle.dump()). This breaks the parallel processing of the books data.
to reproduce:
book = openpyxl.load_workbook('filename.xlsx', read_only=True, data_only=True)
with open('dump.bin', 'wb') as f:
pickle.dump(book, f)
Results with the exception: TypeError: cannot pickle '_io.BufferedReader' object
The 'regular' book (without read_only flag) works fine.