Starts logging messages to files instead of standard error.

def start_logging_to_file(self, program_name=None, log_dir=None):
    """Starts logging messages to files instead of standard error."""
    FLAGS.logtostderr = False

    actual_log_dir, file_prefix, symlink_prefix = find_log_dir_and_names(
        program_name=program_name, log_dir=log_dir)

    basename = '%s.INFO.%s.%d' % (
        file_prefix,
        time.strftime('%Y%m%d-%H%M%S', time.localtime(time.time())),
        os.getpid())
    filename = os.path.join(actual_log_dir, basename)

    if six.PY2:
      self.stream = open(filename, 'a')
    else:
      self.stream = open(filename, 'a', encoding='utf-8')

    # os.symlink is not available on Windows Python 2.
    if getattr(os, 'symlink', None):
      # Create a symlink to the log file with a canonical name.
      symlink = os.path.join(actual_log_dir, symlink_prefix + '.INFO')
      try:
        if os.path.islink(symlink):
          os.unlink(symlink)
        os.symlink(os.path.basename(filename), symlink)
      except EnvironmentError:
        # If it fails, we're sad but it's no error.  Commonly, this
        # fails because the symlink was created by another user and so
        # we can't modify it
        pass