Python Pandas ExcelWriter append to sheet creates a new sheet

I know my code is not a work of art, but I will update this post with the answer to my own question for the sake of completeness, and if anyone stumbles on this post.

It turns out i misunderstood the capabilities of the "append" function in Pandas "pd.ExcelWriter". It is not possible to append to a sheet already existing, the sheet will get overwritten though mode is set to 'a'.

Realizing this i changed my code to build a dataframe for the entire sheet (df_sheet), an then call the "createsheets" function in my code. The first version wrote my data column by column.

"Final" code:

import pandas as pd

# initial files for dataframes
excel_file = 'output.xlsx'
setup_file = 'setup.xlsx'

# write to excel
output_filename = 'output_final.xlsx'
column_name = 0

df = pd.read_excel(excel_file)  # create dataframe of entire sheet
df.columns = df.columns.str.strip().str.lower().str.replace(' ', '_').str.replace('(', '').str.replace(')',
                                                                                                       '')  # clean dataframe titles

df_setup = pd.read_excel(setup_file)
df_setup.columns = df_setup.columns.str.strip().str.lower().str.replace(' ', '_').str.replace('(', '').str.replace(')',
                                                                                                                   '')  # clean dataframe titles

df_2 = pd.merge(df, df_setup)  # Merge data with setup to have krymp size for each wire in dataframe

df_2['wirelabel'] = "'" + df_2['cable'] + "_" + df_2['function_code'] + "-" + df_2['terminal_strip'] + ":" + df_2[
    'terminal']  # creates column for the wirelabel by appending columns with set delimiters. #TODO: delimiters to be by inputs.

df_2.sort_values(by=['switchboard'])  # sort so we get proper order

switchboard_unique = df.switchboard.unique().tolist()  # crate variable containing unique switchboards for printing to excel sheets




def createsheets(output_filename, sheetname, df_towrite):
    with pd.ExcelWriter(output_filename, engine='openpyxl', mode='a') as writer:
        df_towrite.to_excel(writer, sheet_name=sheetname, index=False, header=True)


def to_csv_file(output_filename, df_towrite):
    df_towrite.to_csv(output_filename, mode='w', index=False)

def sorter():
    for s in switchboard_unique:
        df_3 = df_2.loc[df_2['switchboard'] == s]

        krymp_unique = df_3.krymp.unique().tolist()
        krymp_unique.sort()

        column_start = 0
        row_start = 0
        df_sheet = pd.DataFrame([])

        for k in krymp_unique:
            df_5 = df_3.loc[df_3['krymp'] == k]
            df_4 = df_5.filter(['wirelabel'])


            column_name = "krymp " + str(k) + " Tavle: " + str(s)

            df_4 = df_4.rename(columns={"wirelabel": column_name})
            df_4 = df_4.reset_index(drop=True)
            df_sheet = pd.concat([df_sheet, df_4], axis=1)

            column_start = column_start + 1
            row_start = row_start + len(df_5.index) + 1

        createsheets(output_filename, s, df_sheet)
        to_csv_file(s + ".csv", df_sheet)


sorter()

Thank you.