It's simple, just use csv lib (example below):

import csv

import base64

with open('employee_file.csv', mode='w') as employee_file:
    employee_writer = csv.writer(employee_file, delimiter=',', quotechar='"', quoting=csv.QUOTE_MINIMAL)

    employee_writer.writerow(['John Smith', 'Accounting', 'November'])
    employee_writer.writerow(['Erica Meyers', 'IT', 'March'])

Probably you'll need to use StringIO to not save it as a file, but to a string and then to encode to base64 and save it to a binary field. I normally make import export wizards for this. Don't have any example for csv, mostly creating excel files. But the link below will show how it's done.

\Using StringIO and csv 

\https://stackoverflow.com/questions/9157314/how-do-i-write-data-into-csv-format-as-string-not-file

EDIT:

This is an example for a model method, the model needs to have a field named file_field, like in the comment.

Not sure if it'll work just copy paste, because this are just few lines of my complex method. I've just kept the basis

import xlwt
import StringIO
import base64


def create_excel(self, ids)
  tasks = self.env['project.task'].sudo().browse(ids)

  stream = StringIO.StringIO()
  book = xlwt.Workbook(encoding='utf-8')
  sheet = book.add_sheet(u'{}'.format(u'My Sheet'))

  row = 1
  cell = 0

  for task in tasks:
    sheet.write(row, cell, task.name)
    row += 1

  book.save(stream)

  # file_field = fields.Binary()
  self.file_field = base64.encodestring(stream.getvalue())