tensorflow/models · error · NotImplementedError

MaskedRouter is an abstract class that should be subclassed.

Error message

MaskedRouter is an abstract class that should be subclassed.

What it means

Error "MaskedRouter is an abstract class that should be subclassed." thrown in tensorflow/models.

Source

Thrown at official/nlp/modeling/layers/moe.py:233

  Routing using masked matmuls is generally faster than scatter-based routing on
  TPUs.

  Uses Keras add_loss() and add_metric() APIs.
  """

  def _compute_routing_instructions(self, router_probs: tf.Tensor,
                                    expert_capacity: int) -> RouterMask:
    """Computes masks for the top-k experts per token.

    Args:
      router_probs: <float32>[num_groups, tokens_per_group, num_experts]
        probabilities used to determine the routing of tokens to the experts.
      expert_capacity: Each group will send this many tokens to each expert.

    Returns:
      Router mask arrays.
    """
    raise NotImplementedError(
        "MaskedRouter is an abstract class that should be subclassed.")


class ExpertsChooseMaskedRouter(MaskedRouter):
  """Masked matmul router using experts choose tokens assignment.

  This router uses the same mechanism as in Mixture-of-Experts with Expert
  Choice (https://arxiv.org/abs/2202.09368): each expert selects its top
  expert_capacity tokens. An individual token may be processed by multiple
  experts or none at all.

  Note: "experts choose routing" should not be used in decoder blocks because it
  breaks the autoregressive behavior, leading to a mismatch between training
  (teacher forcing) and inference (autoregressive decoding).

  Uses Keras add_loss() and add_metric() APIs.
  """

View on GitHub (pinned to e006f5f0d5)

When it happens

Trigger: Thrown at official/nlp/modeling/layers/moe.py:233 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of tensorflow/models@e006f5f0d5 (2026-08-24). Data as JSON: /api/errors/a889dd71df6a396b. Report an issue: GitHub.