Django Admin : Action for Exporting User details to CSV

admin.py

from django.http import HttpResponse, HttpResponseRedirect
from django.contrib import admin
from django.contrib.auth.admin import UserAdmin

def export_to_csv (modeladmin, request, queryset):
    import csv, StringIO
    from django.core.servers.basehttp import FileWrapper
    
    cols = ['username','email','first_name','last_name']

    # get qs values
    data = list( queryset.values_list(*cols) )

    if not data:
        messages.error(request, 'No data to export')
        return HttpResponseRedirect( request.get_full_path() )

    # create empty csv
    csv_file = StringIO.StringIO()
    csv_writer = csv.writer(csv_file, quoting = csv.QUOTE_ALL)

    # add headers
    csv_writer.writerow( cols )

    # add qs values
    for row in data:
    	csv_writer.writerow( [s.encode('utf-8') for s in row] )

    csv_file.flush()
    csv_file.seek(0)

    response = HttpResponse(FileWrapper( csv_file ), content_type='text/csv')
    response['Content-Disposition'] = "attachment; filename=user-csv-export.csv"

    return response

export_to_csv.short_description = "Export to CSV"

class UserProfileAdmin(UserAdmin):
    actions = [ export_to_csv ]
    # ...

Please my exported csv list comes with a 'b' prepended to each value. How can I fix this?? Also how can I get only unique values from db