title: writerow only write one row
body: <p>how come my writerow code in a for loop only write 1 row into the csv file. My codes are: </p>
  
  <pre><code>for row in announcements_df:
  
      row['Date'] = datetime.datetime.strptime(row['Date'] + row['Time'], '%Y%m%d%H:%M:%S')
      data_writer = csv.DictWriter(open('announcements_date_formated.csv', 'wb'), fieldnames=['asxCode', 'CompanyName', 'Date', 'Time', 'Header', 'page', 'MarketSensitive', 'hour', 'minutes', 'year', 'month', 'day', 'Date2'])
      data_writer.writeheader()
      data_writer.writerow(row)
  </code></pre>
  
  <p>Thank you very much</p>