# Make sure that the __next__() is never called during the
    # get_num_samples call.
    self.assertLen(list(repr_ds), num_samples)

  def test_get_num_samples_returns_none_when_len_raises_error(self):
    class LenRaisingError:
      """A test-only class that raises an error when len() is called.

      This mocks the behavior of an Iterator whose size cannot be determined.