xlsxwriter format for pattern seems... buggy? - xlsxwriter

Question

So I tried to apply a pattern with a format class in xlsxwriter.
import xlsxwriter
wb = xlsxwriter.Workbook("Temp.xlsx")
sheet = wb.add_worksheet()
blank_pattern_format = wb.add_format({
'align':'center',
'valign' : 'vcenter',
'border' : 1,
'shrink' : True,
'font_size' :10,
'bold' : True,
'pattern' : 4,
})
sheet.write("A1","",blank_pattern_format)
wb.close()
I wanted the forth pattern which is "25% gray" in Excel. But the outcome is just solid black for that cell.
The interesting part is, when I check the property of that black cell, it actually shows that it has None color for bg_color, and also "25% gray" pattern is applied to that cell.
I tried to copy the format to other cell in excel. And it turned to the solid black one as well.
But when I got into the cell property again, and chose the exact same pattern("25% gray")(which was shown as already being chosen), and clicked confirm without adjusting anything else, it now shows as 25% gray pattern and not the solid black one.
I think this means there can be a bug in xlsxwriter module for patterns. Is there I can do to fix this case? Or any walk-around maybe? (Documents about pattern in format class was short and not that helpful to fix this case)
Hope you guys can enlighten me. Thanks in advance!

Answer 1

It may be a bug. There are some odd Excel edge conditions when the foreground and background colors aren't specified. I'll look into it.
In the meantime you can work around the issue by specifying a background color:
import xlsxwriter
wb = xlsxwriter.Workbook('temp.xlsx')
sheet = wb.add_worksheet()
blank_pattern_format = wb.add_format({
'align': 'center',
'valign': 'vcenter',
'border': 1,
'shrink': True,
'font_size': 10,
'bold': True,
'pattern': 4,
'bg_color': 'white',
})
sheet.write('A1', '', blank_pattern_format)
wb.close()
Output: