When saving excel file (openpyxl) and then trying to make excel open it on mac get `Permission denied` error

When saving excel file (openpyxl) and then trying to make excel open it on
mac get `Permission denied` error

This code:
workbook.save(outputf)
system('open ' + outputf)
generates this error when run on my mac:
sh: /Users/tylerjw/Desktop/jim_data/August2013_report.xlsx: Permission denied
The file was created with openpyxl. I have not been able to reproduce the
error outside of my application is a tkinter application that is writing a
considerable amount of data to that file.
When I run similar code in windows it doesn't error, it opens excel with
the file. The only difference is the absence of the open command.
What could case this error?