title: Python TimedRotatingFileHandler overwrites logs
body: <p>I setup <code>TimedRotatingFileHandler</code> like that:</p>
  
  <pre><code>import logging as logging
  from logging.handlers import TimedRotatingFileHandler
  import os
  import time
  
  logger = logging.getLogger()
  logger.setLevel(logging.DEBUG)
  
  # new file every minute
  rotation_logging_handler = TimedRotatingFileHandler('logs/log', 
                                 when='m', 
                                 interval=1, 
                                 backupCount=5)
  rotation_logging_handler.setLevel(logging.DEBUG)
  
  format = u'%(asctime)s\t%(levelname)s\t%(filename)s:%(lineno)d\t%(message)s'
  rotation_logging_handler.setFormatter(logging.Formatter(format))
  rotation_logging_handler.suffix = '%Y-%m-%d'
  
  logger.addHandler(rotation_logging_handler)
  </code></pre>
  
  <p>Usage:</p>
  
  <pre><code>logger.logging.info('Service started at port %s', config.get_param('port'))
  
  while True:
      time.sleep(21)
      logger.logging.info('Now time is {}'.format(time.time()))
  </code></pre>
  
  <p>I expected that every minute new messages from <code>logs/log</code> had to append to existing log file for current date. Instead it every minute messages from <code>logs/log</code> overwrote existing log file for current date.</p>
  
  <p>What should I do to reach that behaviour?</p>
  
  <p>PS: After small research I found that <code>TimedRotatingFileHandler</code> in the <code>doRollover</code> method deletes existing log file and creates new file. So first solution is to create new handler derived from <code>TimedRotatingFileHandler</code> 
   which creates new file (with some index for example) insted of deleting existing log file.</p>