Pandas to excel bold text

Thanks to @NoobVB I was pointed in the right direction: write_rich_string. Found this great article describing it as a cookbook and turned it into a working solution:

import pandas as pd
from pandas.io.excel._xlsxwriter import XlsxWriter


class RichExcelWriter(XlsxWriter):
    def __init__(self, *args, **kwargs):
        super(RichExcelWriter, self).__init__(*args, **kwargs)

    def _value_with_fmt(self, val):
        if type(val) == list:
            return val, None
        return super(RichExcelWriter, self)._value_with_fmt(val)

    def write_cells(self, cells, sheet_name=None, startrow=0, startcol=0, freeze_panes=None):
        sheet_name = self._get_sheet_name(sheet_name)
        if sheet_name in self.sheets:
            wks = self.sheets[sheet_name]
        else:
            wks = self.book.add_worksheet(sheet_name)
            #add handler to the worksheet when it's created
            wks.add_write_handler(list, lambda worksheet, row, col, list, style: worksheet._write_rich_string(row, col, *list))
            self.sheets[sheet_name] = wks
        super(RichExcelWriter, self).write_cells(cells, sheet_name, startrow, startcol, freeze_panes)

writer = RichExcelWriter('sample.xlsx')
workbook = writer.book
bold = workbook.add_format({'bold': True})

df = pd.DataFrame([["Hello world."], [["Hello", bold, "bold", "world."]], columns = ["example"])

df.to_excel(writer, sheet_name='Sample', index=False)
writer.save()

This is the easiest way I figured out so far.

An example of converting a Pandas dataframe to an Excel file with a user defined header format using Pandas and XlsxWriter.

##############################################################################
#
# An example of converting a Pandas dataframe to an xlsx file
# with a user defined header format.
#
# SPDX-License-Identifier: BSD-2-Clause
# Copyright 2013-2022, John McNamara, 
#

import pandas as pd

# Create a Pandas dataframe from some data.
data = [10, 20, 30, 40, 50, 60]
df = pd.DataFrame({'Heading': data,
                   'Longer heading that should be wrapped' : data})

# Create a Pandas Excel writer using XlsxWriter as the engine.
writer = pd.ExcelWriter("pandas_header_format.xlsx", engine='xlsxwriter')

# Convert the dataframe to an XlsxWriter Excel object. Note that we turn off
# the default header and skip one row to allow us to insert a user defined
# header.
df.to_excel(writer, sheet_name='Sheet1', startrow=1, header=False)

# Get the xlsxwriter workbook and worksheet objects.
workbook  = writer.book
worksheet = writer.sheets['Sheet1']

# Add a header format.
header_format = workbook.add_format({
    'bold': True,
    'text_wrap': True,
    'valign': 'top',
    'fg_color': '#D7E4BC',
    'border': 1})

# Write the column headers with the defined format.
for col_num, value in enumerate(df.columns.values):
    worksheet.write(0, col_num + 1, value, header_format)

# Close the Pandas Excel writer and output the Excel file.
writer.save()

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