If you are unable to insert new worksheets into an Excel workbook or make other changes to the workbook because it is protected and you don't know the password to unprotect the workbook, if the workbook is in a .xlsx file, you can remove the password by changing the .xlsx file extension to .zip, then extract the contents of the zip file, locate the
workbook.xmlfile within it and remove the password between the double quotes following
workbookProtection workbookPassword=using a text editor, then resave the file and recompress the directory produced when you unzipped the zip file, but, on an OS X system, you will need to ensure that the directory and its contents are compressed in the way Excel is expecting, e.g. with the zipdir Pyhon script.
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