def _maybe_read_file(filename):
  """Read the given file, if it exists.

  Args:
    filename: A path to a file.

  Returns:
    A string containing the file contents, or `None` if the file does
    not exist.
  """
  try:
    with open(filename) as infile:
      return infile.read()
  except IOError as e:
    if e.errno == errno.ENOENT:
      return None
Read the given file, if it exists.

  Args:
    filename: A path to a file.

  Returns:
    A string containing the file contents, or `None` if the file does
    not exist.