def hash_file(hash_obj, path):
  """Hash the contents of a file.

  Args:
    hash_obj: an open hash object, e.g. hashlib.sha1().
    path: path to the file to hash.

  Returns:
    The passed-in hash_obj.
  """
  with open(path, 'rb') as f:
    while True:
      b = f.read(8192)
      if not b:
        break
      hash_obj.update(b)
  return hash_obj