{"record":{"id":"e553604302ca5b8e","repo":"keras-team/keras","slug":"got-no-xs-to-scan-over-and-length-not-provided-e55360","errorCode":null,"errorMessage":"Got no `xs` to scan over and `length` not provided.","messagePattern":"Got no `xs` to scan over and `length` not provided\\.","errorType":"validation","errorClass":"ValueError","httpStatus":null,"severity":"error","filePath":"keras/src/backend/tensorflow/core.py","lineNumber":295,"sourceCode":"        return tf.map_fn(f, xs, fn_output_signature=fn_output_signature)\n    else:\n        fn_output_signature = get_fn_output_signature(xs[0])\n        return tf.map_fn(f, xs, fn_output_signature=fn_output_signature)\n\n\ndef scan(f, init, xs=None, length=None, reverse=False, unroll=1):\n    # We have reimplemented `scan` to match the behavior of `jax.lax.scan`\n    # Ref: tf.scan, jax.lax.scan\n    if not callable(f):\n        raise TypeError(f\"`f` should be a callable. Received: f={f}\")\n    if not isinstance(unroll, bool):\n        if not isinstance(unroll, int) or unroll < 1:\n            raise ValueError(\n                \"`unroll` must be an positive integer or boolean. \"\n                f\"Received: unroll={unroll}\"\n            )\n    if xs is None and length is None:\n        raise ValueError(\"Got no `xs` to scan over and `length` not provided.\")\n\n    input_is_sequence = tree.is_nested(xs)\n    output_is_sequence = tree.is_nested(init)\n\n    def pack_input(x):\n        return tree.pack_sequence_as(xs, x) if input_is_sequence else x[0]\n\n    def pack_output(x):\n        return tree.pack_sequence_as(init, x) if output_is_sequence else x[0]\n\n    if xs is None:\n        xs_flat = []\n        n = int(length)\n    else:\n        # xs_flat = flatten_input(xs)\n        xs_flat = tree.flatten(xs)\n        xs_flat = [tf.convert_to_tensor(elem) for elem in xs_flat]\n        n = int(length) if length is not None else tf.shape(xs_flat[0])[0]","sourceCodeStart":277,"sourceCodeEnd":313,"githubUrl":"https://github.com/keras-team/keras/blob/7a34a03db60bf60042242d6a556fc3be119046a5/keras/src/backend/tensorflow/core.py#L277-L313","documentation":"Error \"Got no `xs` to scan over and `length` not provided.\" thrown in keras-team/keras.","triggerScenarios":"Thrown at keras/src/backend/tensorflow/core.py:295 when the library encounters an invalid state.","commonSituations":"See trigger scenarios.","solutions":[],"exampleFix":null,"handlingStrategy":null,"validationCode":null,"typeGuard":null,"tryCatchPattern":null,"preventionTips":[],"tags":[],"backgroundTag":null,"analyzedSha":"7a34a03db60bf60042242d6a556fc3be119046a5","analyzedAt":"2026-08-25T21:25:25.994Z","schemaVersion":2},"datasetVersion":"2026-08-26T02:17:13.382Z"}