keras-team/keras · error · ValueError

Got no `xs` to scan over and `length` not provided.

Error message

Got no `xs` to scan over and `length` not provided.

What it means

Error "Got no `xs` to scan over and `length` not provided." thrown in keras-team/keras.

Source

Thrown at keras/src/backend/tensorflow/core.py:295

        return tf.map_fn(f, xs, fn_output_signature=fn_output_signature)
    else:
        fn_output_signature = get_fn_output_signature(xs[0])
        return tf.map_fn(f, xs, fn_output_signature=fn_output_signature)


def scan(f, init, xs=None, length=None, reverse=False, unroll=1):
    # We have reimplemented `scan` to match the behavior of `jax.lax.scan`
    # Ref: tf.scan, jax.lax.scan
    if not callable(f):
        raise TypeError(f"`f` should be a callable. Received: f={f}")
    if not isinstance(unroll, bool):
        if not isinstance(unroll, int) or unroll < 1:
            raise ValueError(
                "`unroll` must be an positive integer or boolean. "
                f"Received: unroll={unroll}"
            )
    if xs is None and length is None:
        raise ValueError("Got no `xs` to scan over and `length` not provided.")

    input_is_sequence = tree.is_nested(xs)
    output_is_sequence = tree.is_nested(init)

    def pack_input(x):
        return tree.pack_sequence_as(xs, x) if input_is_sequence else x[0]

    def pack_output(x):
        return tree.pack_sequence_as(init, x) if output_is_sequence else x[0]

    if xs is None:
        xs_flat = []
        n = int(length)
    else:
        # xs_flat = flatten_input(xs)
        xs_flat = tree.flatten(xs)
        xs_flat = [tf.convert_to_tensor(elem) for elem in xs_flat]
        n = int(length) if length is not None else tf.shape(xs_flat[0])[0]

View on GitHub (pinned to 7a34a03db6)

When it happens

Trigger: Thrown at keras/src/backend/tensorflow/core.py:295 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of keras-team/keras@7a34a03db6 (2026-08-25). Data as JSON: /api/errors/e553604302ca5b8e. Report an issue: GitHub.