Writing "inf" silently creates invalid workbook
*Created originally on Bitbucket by [snorfalorpagus (Joshua Arnott)](https://bitbucket.org/%7B60afeab2-3636-4f2e-bd58-aefc874ab62f%7D/)* The following code will run without exception, but creates an invalid workbook. This is caused because Excel does not appear to have a representation of infinity. The same problem occurs when trying to write a `numpy.inf` value. I'm not sure what the correct behavior is here: either raise an exception, or write "inf" as a string. ``` #!python import openpyxl wb = openpyxl.Workbook() sheet = wb.create_sheet("Test") sheet["A1"] = float("inf") wb.save("test.xlsx") ``` Tested on openpyxl 2.4.8 and 2.5.0b1, both installed via anaconda/conda-forge.
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