openpyxl: '#' is inserted to formula when saving to file - excel-formula

Question

When I add the following formula to a cell, the cell's value looks good when printed to the console. However, after I save the file, the formula has '#' inserted right after the '=' (for simplicity, I am providing the output from the console):
>>> from openpyxl import Workbook
>>> wb = Workbook()
>>> ws = wb.active
>>> ws['A1'] = '=CONCAT("Week ",TEXT(MID(' + get_column_letter(9) + '1,6,2)+ 1, "##"))'
>>> ws['A1'].value
'=CONCAT("Week ",TEXT(MID(I1,6,2)+ 1, "##"))'
>>> wb.save('formula.xlsx')
>>>
In the 'formula.xlsx' file, the formula looks like this:
=#CONCAT("Week ",TEXT(MID(I1,6,2)+ 1, "##"))
If, however, instead of '=CONCAT()' I specify '=SUM()', for example, it is saved as expected, i.e. without the '#' inserted.
I am using openpyxl 3.0.3 and Python 3.8.
Many thanks
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I have looked into the XML code of 'formula.xlsx'; but before doing that, I opened it in Excel, copied cell A1 into cell D1, and deleted '#' from the formula in cell D1, after which D1 started showing the correct value while A1 still showed the '#NAME?' error.
So, after my changes in cell D1, the XML code for the sheet showed the following:
<row r="1" spans="1:9" x14ac:dyDescent="0.45">
<c r="A1" t="e"><f ca="1">_xludf.CONCAT("Week ",TEXT(MID(I1,6,2)+ 1, "##"))</f><v>#NAME?</v></c>
<c r="D1" t="str"><f>_xlfn.CONCAT("Week ",TEXT(MID(I1,6,2)+ 1, "##"))</f><v>Week 68</v></c>
<c r="I1"><v>12345678</v></c>
</row>
The _xludf prefix used by openpyxl for CONCAT in cell A1 above is described as "User Defined Function" on https://learn.microsoft.com/en-us/office/client-developer/excel/xludf.
Could it mean that the library did not recognise CONCAT as a standard Excel function, and therefore used _xludf instead of _xlfn for it?
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Answer 1

As specified in the openpyxl documentation known formulas are used just by inserting the formula name.
One can use
>>> from openpyxl.utils import FORMULAE
>>> "CONCAT" in FORMULAE
False
To check if the formula is a known one in openpyxl. If the formula isn't you need to add _xlfn. just before the formula name, like so:
>>> ws['A1'] = '=_xlfn.CONCAT("Week ",TEXT(MID(' + get_column_letter(9) + '1,6,2)+ 1, "##"))
It is also mentioned in the documentation:
If you’re trying to use a formula that isn’t known this could be
because you’re using a formula that was not included in the initial
specification. Such formulae must be prefixed with _xlfn. to work.

Answer 2

I have the same problem using formulae in Spanish (I'm from Argentina).
When I try to assign something like "=SUMA(A1:A20)" to a cell, it comes out as "=#SUMA(A1:A20)".
I tried the _xlfn. solution, but now it just ends up as
"=#_xlfn.SUMA(A1:A20)"
If/when I find an answer I'll post it here.
SOLVED
If you are using a non-English version of Excel, you still have to assign the cell the English name of the function, e.g. "=SUM(A1:A20)"
Afterwards, when you check the content of the cell in the worksheet, it will have changed to the proper language, in this case the Spanish "=SUMA(A1:A20)"
Caveat: I've only checked it in the Spanish version, but I'm pretty sure it works for all.
Also: If you use another set of characters as separators, for example a comma (,) instead of a dot (.) for decimals, you still need to use the dot when assigning a formula to a cell, for example "= E8 * 0.5". You will see a comma when you check the cell. Using a comma in that string will result in a damaged-file error when opening the xlsx file.

Answer 3

In my case I was trying to put =ARRED formulae and the character # appears in the Excel file like =#ARRED.
ARRED is a Brazilian Portuguese prefix and is not recognized, so I replaced by ROUND and is worked perfectly.
Testing:
from openpyxl.utils import FORMULAE
"ARRED" in FORMULAE
False
"ROUND" in FORMULAE
True

Answer 4

Specifying _xlfn prefix explicitly in the python code fixes the problem:
>>> ws['A1'] = '=_xlfn.CONCAT("Week ",TEXT(MID(' + get_column_letter(9) + '1,6,2)+ 1, "##"))'
Thanks goes to Dror Av. for guidance!