Adding # to formula after = - formula

Question

When I add the formula FORECAST.ETS, it adds an # after the equal symbol, like this: = #FORECAST.ETS. Why is this happening?
The code snippet is:
ws.cell(column=1, row=2, value="=FORECAST.ETS(...)"
When I open it with Excel (latest Office 365 version), it shows as =#FORECAST.ETS(..)

Answer 1

I have hit the same issue, but not with Python and openpyxl, but with dotnet Core C# and EPPLUS. What follows is perhaps a workaround based on my findings... but not ideal. I suspect it will work with openpyxl too.
Re-creating the problem
I have written a simplified C# console app that firstly creates a new XLSX (foo.xlsx), writes out some data and my formula, and then outputs the cell with the formula and the value to the Console. It then saves and closes the XLSX, and reopens it and again outputs the formula cell and its value. The code is as follows:
using OfficeOpenXml;
using System;
using System.IO;
namespace TestFormula
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Test starts");
ExcelPackage.LicenseContext = LicenseContext.NonCommercial;
if (File.Exists($".\\foo.xlsx"))
{
File.Delete($".\\foo.xlsx");
}
using (var ep = new ExcelPackage(new FileInfo($".\\foo.xlsx")))
{
ExcelWorkbook wb = ep.Workbook;
ExcelWorksheet wsTest = null;
wsTest = wb.Worksheets.Add("Test");
// Add some look up data...
for (int row = 1; row <= 5; row++)
{
wsTest.Cells[row, 1].Value = row;
wsTest.Cells[row, 2].Value = $"Name {row}";
}
wsTest.Cells[1, 4].Formula = $"=XLOOKUP($A3,$A:$A,$B:$B))";
Console.WriteLine($"Add: formula=\"{wsTest.Cells[1, 4].Formula}\"");
Console.WriteLine($"Add: value=\"{wsTest.Cells[1, 4].Value}\"");
ep.Save();
ep.Dispose();
}
using (var ep = new ExcelPackage(new FileInfo($".\\foo.xlsx")))
{
ExcelWorkbook wb = ep.Workbook;
ExcelWorksheet wsTest = null;
wsTest = wb.Worksheets["Test"];
Console.WriteLine($"Open: formula=\"{wsTest.Cells[1, 4].Formula}\"");
Console.WriteLine($"Open: value=\"{wsTest.Cells[1, 4].Value}\"");
ep.Dispose();
}
Console.WriteLine("Test ends");
}
}
}
The output from the above looks like this...
Note that the formula after closing and re-opening the XLSX with EPPLUS reads just as it was written.
However, if I open the file with Excel I can see that an # has been inserted after the = sign.
If I then double click on the formula cell, I get an Excel error message...
I answered "no" to this question because I wanted to continue to experiment with what was happening behind the scenes.
After double clicking the formula cell to edit it, when I now hit ENTER with the # in the formula, it works. At this point I save the XLSX with the change made.
If I now delete some of my code and just run...
using OfficeOpenXml;
using System;
using System.IO;
namespace TestFormula
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Test starts");
ExcelPackage.LicenseContext = LicenseContext.NonCommercial;
using (var ep = new ExcelPackage(new FileInfo($".\\foo.xlsx")))
{
ExcelWorkbook wb = ep.Workbook;
ExcelWorksheet wsTest = null;
wsTest = wb.Worksheets["Test"];
Console.WriteLine($"Open: formula=\"{wsTest.Cells[1, 4].Formula}\"");
Console.WriteLine($"Open: value=\"{wsTest.Cells[1, 4].Value}\"");
ep.Dispose();
}
Console.WriteLine("Test ends");
}
}
}
I get the following output...
What's particularly interesting about the output is that the formula has been modified by Excel and has been prefixed with _alfn.SINGLE.
Research
It is worth declaring here that I am running Microsoft 365 and I always have patches and updates automatically applied as soon as they become available. So my version of Excel is the latest version.
Google-ing for _alfn.SINGLE provides a number of hits (see References below) and from these I have concluded the following:
In Aug 2019 Microsoft released an update that introduced a new formula keyword called XLOOKUP... intended to replace VLOOKUP and HLOOKUP. As such, the XLSX file format was updated to allow for this new feature. The second reference below mentions dates of the introduction of other formulas around Sep 2018.
I'm guessing that the EPLUS library (and probably the openpyxl) have not updated their file format to compensate for the addition of these new/changed features.
When Excel opens an older file version and detects a more recent formula keyword (i.e. a keyword that was not available in the earlier file version), it does not automatically resolve the formula, but instead throws the error I mentioned above, and then resolves the problem by prefixing the new formula keyword with _alfn.SINGLE.
Solution
It's dirty and short term until the EPPLUS/openpyxl libraries catch up. In my case, in code simply replace...
wsTest.Cells[1, 4].Formula = $"=XLOOKUP($A3,$A:$A,$B:$B)";
... with ...
wsTest.Cells[1, 4].Formula = $"_xlfn.SINGLE(_xlfn.XLOOKUP($A3,$A:$A,$B:$B))";
References
Issue: An _xlfn. prefix is displayed in front of a formula by Microsoft
XLOOKUP XMATCH FILTER RANDARRAY SEQUENCE SORT SORTBY UNIQUE CONCAT IFS MAXIFS MINIFS SWITCH TEXTJOIN by Andreas Killer