Apply format to a cell after being written in XlsxWriter

Another workaround is to use conditional_format, and use type='no_errors':

worksheet.conditional_format(your_range, {'type': 'no_errors',
                                          'format': your_format})

One way of doing that - using one wrapper method to write cell, and helper method to overwrite cell's value and style

import xlsxwriter

class XLSGenerator:
    def __init__(self):
        self.workbook = xlsxwriter.Workbook('file.xls')
        sheet1 = self.workbook.add_worksheet('sheet1')
        sheet2 = self.workbook.add_worksheet('sheet2')
        self.sheets = {'sheet1': sheet1, 'sheet2': sheet2}
        #  dictionary with all written cells
        self.written_cells = {sheet: {} for sheet in self.sheets}

    def write_cell(self, sheet_name, cell, value, cell_format_dict=None):
        """Writes value and style, and saves it in self.written_cells"""

        sheet = self.sheets[sheet_name]
        if cell_format_dict:
            cell_format = self.workbook.add_format(cell_format_dict)
            sheet.write(cell, value, cell_format)
        else:
            cell_format_dict = None
            sheet.write(cell, value)

        # save sheet_name, cell and cell_value, and cell_format (dict)
        # example ['sheet1']['C12'] = ('some_text', {'font_size': 14, 'bold': True}
        self.written_cells[sheet_name][cell] = (value, cell_format_dict)

    def apply_style(self, sheet_name, cell, cell_format_dict):
        """Apply style for any cell, with value or not. Overwrites cell with joined 
        cell_format_dict and existing format and with existing or blank value"""

        written_cell_data = self.written_cells[sheet_name].get(cell)
        if written_cell_data:
            existing_value, existing_cell_format_dict = self.written_cells[sheet_name][cell]
            updated_format = dict(existing_cell_format_dict or {}, **cell_format_dict)
        else:
            existing_value = None
            updated_format = cell_format_dict

        self.write_cell(sheet_name, cell, existing_value, updated_format)

Usage like this

generator = XLSGenerator()
generator.write_cell('sheet1', 'A1', '10')
generator.write_cell('sheet1', 'B2', '20')
generator.write_cell('sheet1', 'C3', '30')

table_borders = {"left": 1, 'right': 1, 'top': 1, 'bottom': 1}
for cell in ('A1', 'A2', 'A3', 'B1', 'B2', 'B3', 'C1', 'C2', 'C3'):
   generator.apply_style('sheet1', cell, table_borders)

generator.workbook.close()

I'm the author of that module and unfortunately that isn't possible.

It is a planned feature, and (a small) part of the internal infrastructure is there to support it, but it isn't currently available and I can't say when it will be.

Update: this feature was never implemented and is no longer planned.