Apparently, it comes from a function in a file named formatting.py in xlrd source, wihch tests wether a string is a date formatting string. I think you should ignore the warning.
Gribouillis
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@gribouillis thank you for the answer, but how did you figure it out?
I wrote excel warning format produces constant result in google's search form and in the 2nd page of google results, there was this link http://nullege.com/codes/show/src@xlrd-0.7.1@xlrd@formatting.py/33/struct.unpack which points to a version of xlrd's source code, which contains the line fprintf(book.logfile,
"WARNING *** format %r produces constant result\n",
fmt)
:)
Gribouillis
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