Xlsxwriter: TypeError: “expected string or buffer”

Question

I have that exception, as described in the title, when trying to generate an excel file from a model which is filtered by a query. The query works as expected and gives me the right and complete results. However, the big deal occurs when I try to, actually, generate the excel file:

  File "/home/luismasuelli/Proyectos/CentralCar/main/views.py", line 100, in export_xls
    workbook.close()
  File "/home/luismasuelli/.virtualenvs/py27dj15/local/lib/python2.7/site-packages/xlsxwriter/workbook.py", line 237, in close
    self._store_workbook()
  File "/home/luismasuelli/.virtualenvs/py27dj15/local/lib/python2.7/site-packages/xlsxwriter/workbook.py", line 405, in _store_workbook
    xml_files = packager._create_package()
  File "/home/luismasuelli/.virtualenvs/py27dj15/local/lib/python2.7/site-packages/xlsxwriter/packager.py", line 139, in _create_package
    self._write_shared_strings_file()
  File "/home/luismasuelli/.virtualenvs/py27dj15/local/lib/python2.7/site-packages/xlsxwriter/packager.py", line 265, in _write_shared_strings_file
    sst._assemble_xml_file()
  File "/home/luismasuelli/.virtualenvs/py27dj15/local/lib/python2.7/site-packages/xlsxwriter/sharedstrings.py", line 53, in _assemble_xml_file
    self._write_sst_strings()
  File "/home/luismasuelli/.virtualenvs/py27dj15/local/lib/python2.7/site-packages/xlsxwriter/sharedstrings.py", line 83, in _write_sst_strings
    self._write_si(string)
  File "/home/luismasuelli/.virtualenvs/py27dj15/local/lib/python2.7/site-packages/xlsxwriter/sharedstrings.py", line 95, in _write_si
    string = re.sub('(_x[0-9a-fA-F]{4}_)', r'_x005F\1', string)
  File "/home/luismasuelli/.virtualenvs/py27dj15/lib/python2.7/re.py", line 151, in sub
    return _compile(pattern, flags).sub(repl, string, count)
TypeError: expected string or buffer

What could be causing me that exception? I tried to change the encoding to put utf8 as suggested but that doesn't help me here.

I'm working with Xlsxwriter 0.5.2 / python 2.7.4 (ubuntu) from a django app, as follows:

# -*- coding: utf8 -*-
#...
from StringIO import StringIO
#...
import datetime
#...
from django.http import HttpResponse, Http404
#...
import xlsxwriter
#...
from main.models import Contact

#...

def export_xls(request, period=''):

    deltas = {
        'D': datetime.timedelta(days=1),
        'W': datetime.timedelta(days=7),
        'M': datetime.timedelta(days=30),
        '6M': datetime.timedelta(days=182),
        'Y': datetime.timedelta(days=365)
    }

    query = Contact.objects.all().order_by('-enviado_en')

    if period:
        query = query.filter(enviado_en__gt=datetime.datetime.now() - deltas[period])

    stream = StringIO()
    workbook = xlsxwriter.Workbook(stream, {'in_memory': True})
    worksheet = workbook.add_worksheet('Contactos')
    #poner encabezados
    header_format = workbook.add_format()
    header_format.set_align("center")
    header_format.set_bold()
    header_format.set_font_name('Arial')
    worksheet.set_column(9, 9, 40)
    worksheet.write_string(0, 0, Contact._meta.get_field('enviado_en').verbose_name, header_format)
    worksheet.write_string(0, 1, Contact._meta.get_field('nombre').verbose_name, header_format)
    worksheet.write_string(0, 2, Contact._meta.get_field('apellido').verbose_name, header_format)
    worksheet.write_string(0, 3, Contact._meta.get_field('email').verbose_name, header_format)
    worksheet.write_string(0, 4, Contact._meta.get_field('telefono').verbose_name, header_format)
    worksheet.write_string(0, 5, Contact._meta.get_field('ciudad').verbose_name, header_format)
    worksheet.write_string(0, 6, Contact._meta.get_field('vehiculo').verbose_name, header_format)
    worksheet.write_string(0, 7, Contact._meta.get_field('kilometraje').verbose_name, header_format)
    worksheet.write_string(0, 8, Contact._meta.get_field('dia_preferente').verbose_name, header_format)
    worksheet.write_string(0, 9, Contact._meta.get_field('mensaje').verbose_name, header_format)
    #poner cada fila
    body_format = workbook.add_format()
    body_format.set_font_name('Arial')
    current_row = 1
    for contact in query:
        worksheet.write(current_row, 0, contact.enviado_en.strftime("%d/%m/%Y %H/%M/%S"), body_format)
        worksheet.write(current_row, 1, contact.nombre, body_format)
        worksheet.write(current_row, 2, contact.apellido, body_format)
        worksheet.write(current_row, 3, contact.email, body_format) 
        worksheet.write(current_row, 4, contact.telefono, body_format) 
        worksheet.write(current_row, 5, contact.ciudad, body_format) 
        worksheet.write(current_row, 6, contact.get_vehiculo_display(), body_format)
        worksheet.write(current_row, 7, contact.get_kilometraje_display(), body_format) 
        worksheet.write(current_row, 8, contact.get_dia_preferente_display(), body_format) 
        worksheet.write(current_row, 9, contact.mensaje, body_format)
        current_row += 1
    workbook.close()
    data = stream.getvalue()
    response = HttpResponse(content=data, content_type='application/vnd.ms-excel')
    response['Content-Disposition'] = 'attachment; filename=reporte-%s.xlsx' % datetime.datetime.now().strftime("%Y%m%d%H%M%S")
    return response

Being my model Contact:

class Contact(models.Model):

    DIAS = (
        #('?', 'Cualquiera'),
        ('L', 'Lunes'),
        ('M', 'Martes'),
        ('I', 'Miercoles'),
        ('J', 'Jueves'),
        ('V', 'Viernes'),
        ('S', 'Sábado')
    )
    KILOMETRAJE = (
        #('??', '(No especifica)'),
        ('00', '0km'),
        ('05', '1 - 5000km'),
        ('10', '5000km - 10000km'),
        ('15', '10001km - 15000km'),
        ('20', '15001km - 20000km'),
        ('25', '20001km - 25000km'),
        ('30', '25001km - 30000km'),
        ('35', '30001km - 35000km'),
        ('40', '35001km - 40000km'),
        ('45', '40001km - 45000km'),
        ('50', '45001km - 50000km'),
        ('55', '50001km - 55000km'),
        ('60', '55001km - 60000km'),
        ('65', '60001km - 65000km'),
        ('70', '65001km - 70000km'),
        ('75', '70001km - 75000km'),
        ('80', '75001km - 80000km'),
        ('85', '80001km - 85000km'),
        ('90', '85001km - 90000km'),
        ('95', '90001km - 950000km'),
        ('XX', '95001km - 100000km'),
        ('++', 'Más de 100000km')
    )

    enviado_en = models.DateTimeField(default=tznow, null=False, db_index=True, verbose_name=_(u"Enviado en"))
    #campos de contacto
    nombre = models.CharField(max_length=30, null=False, blank=False, db_index=True, verbose_name=_(u"Nombre"))
    apellido = models.CharField(max_length=30, null=False, blank=False, db_index=True, verbose_name=_(u"Apellido"))
    email = models.EmailField(null=False, blank=False, db_index=True, verbose_name=_(u"E-mail"))
    telefono = models.CharField(max_length=10, null=False, blank=False, db_index=True, validators=[contact_phone_number_format], verbose_name=_(u"Teléfono"))
    ciudad = models.CharField(max_length=20, null=False, blank=False, validators=[validate_ciudad], verbose_name=_(u"Ciudad"))
    #campos solamente de cotizacion
    vehiculo = models.CharField(max_length=20, blank=True, db_index=True, choices=vehicle_choices(), verbose_name=_(u"Vehículo"))
    kilometraje = models.CharField(max_length=2, blank=True, db_index=True, choices=KILOMETRAJE, default='05', verbose_name=_(u"Kilometraje"))
    #campos solamente de agendar cita
    dia_preferente = models.CharField(max_length=1, blank=True, choices=DIAS, default='L', verbose_name=_(u'Día para cita'))
    #mensaje
    mensaje = models.TextField(max_length=4095, null=False, blank=False, verbose_name=_(u"Mensaje"))

assume vehicle_choices() exists and returns a valid choice set for the input field.

Answer 1

I found the answer: verbose_name objects are, actually, proxies returned from ugettext_lazy. calling force_unicode(ugl_obj) converts them to string (conversion is not done automagically).

in my case, each value in the header printing was wrapped, eg:

    worksheet.write_string(0, 0, force_unicode(Contact._meta.get_field('enviado_en').verbose_name), header_format)