Iterate through columns in Read-only workbook in openpyxl

Question

I have a somewhat large .xlsx file - 19 columns, 5185 rows. I want to open the file, read all the values in one column, do some stuff to those values, and then create a new column in the same workbook and write out the modified values. Thus, I need to be able to both read and write in the same file.

My original code did this:

def readExcel(doc):
    wb = load_workbook(generalpath + exppath + doc)
    ws = wb["Sheet1"]

    # iterate through the columns to find the correct one
    for col in ws.iter_cols(min_row=1, max_row=1):
        for mycell in col:
            if mycell.value == "PerceivedSound.RESP":
                origCol = mycell.column

    # get the column letter for the first empty column to output the new values
    newCol = utils.get_column_letter(ws.max_column+1)

    # iterate through the rows to get the value from the original column,
    # do something to that value, and output it in the new column
    for myrow in range(2, ws.max_row+1):
        myrow = str(myrow)
        # do some stuff to make the new value
        cleanedResp = doStuff(ws[origCol + myrow].value)
        ws[newCol + myrow] = cleanedResp

    wb.save(doc)

However, python threw a memory error after row 3853 because the workbook was too big. The openpyxl docs said to use Read-only mode (https://openpyxl.readthedocs.io/en/latest/optimized.html) to handle big workbooks. I'm now trying to use that; however, there seems to be no way to iterate through the columns when I add the read_only = True param:

def readExcel(doc):
    wb = load_workbook(generalpath + exppath + doc, read_only=True)
    ws = wb["Sheet1"]

    for col in ws.iter_cols(min_row=1, max_row=1):
        #etc.

python throws this error: AttributeError: 'ReadOnlyWorksheet' object has no attribute 'iter_cols'

If I change the final line in the above snippet to:

for col in ws.columns:

python throws the same error: AttributeError: 'ReadOnlyWorksheet' object has no attribute 'columns'

Iterating over rows is fine (and is included in the documentation I linked above):

for col in ws.rows:

(no error)

This question asks about the AttritubeError but the solution is to remove Read-only mode, which doesn't work for me because openpyxl won't read my entire workbook in not Read-only mode.

So: how do I iterate through columns in a large workbook?

And I haven't yet encountered this, but I will once I can iterate through the columns: how do I both read and write the same workbook, if said workbook is large?

Thanks!

Answer 1

According to the documentation, ReadOnly mode only supports row-based reads (column reads are not implemented). But that's not hard to solve:

wb2 = Workbook(write_only=True)
ws2 = wb2.create_sheet()

# find what column I need
colcounter = 0
for row in ws.rows:
    for cell in row:
        if cell.value == "PerceivedSound.RESP":
            break
        colcounter += 1
    
    # cells are apparently linked to the parent workbook meta
    # this will retain only values; you'll need custom
    # row constructor if you want to retain more

    row2 = [cell.value for cell in row]
    ws2.append(row2) # preserve the first row in the new file
    break # stop after first row

for row in ws.rows:
    row2 = [cell.value for cell in row]
    row2.append(doStuff(row2[colcounter]))
    ws2.append(row2) # write a new row to the new wb
    
wb2.save('newfile.xlsx')
wb.close()
wb2.close()

# copy `newfile.xlsx` to `generalpath + exppath + doc`
# Either using os.system,subprocess.popen, or shutil.copy2()

You will not be able to write to the same workbook, but as shown above you can open a new workbook (in writeonly mode), write to it, and overwrite the old file using OS copy.