What causes "No items to encode" in nautechsystems/nautilus_trader?

Guard the call site: skip encoding (or produce an empty batch another way) when the collection is empty.

No items to encode

Error message

No items to encode

What it means

The Arrow encode function for custom data requires at least one item: it inspects the first item to discover the Python `encode_record_batch_py` method. If the caller passes an empty slice, this error is returned. It is a guard against encoding a zero-length batch through the Python-side encoder.

Source

Thrown at crates/model/src/python/data/mod.rs:388

        Ok(Arc::new(wrapper) as Arc<dyn crate::data::CustomDataTrait>)
    })
}

/// Encodes `CustomData` items to `RecordBatch` via Python `encode_record_batch_py`.
#[allow(unsafe_code)]
#[cfg(all(feature = "python", feature = "arrow"))]
fn py_encode_custom_data_to_record_batch(
    items: &[std::sync::Arc<dyn crate::data::CustomDataTrait>],
) -> Result<arrow::record_batch::RecordBatch, anyhow::Error> {
    pyo3::Python::attach(|py| {
        let py_items: Result<Vec<_>, _> = items.iter().map(|item| item.to_pyobject(py)).collect();
        let py_items = py_items.map_err(|e| anyhow::anyhow!("Failed to convert to Python: {e}"))?;
        let py_list = pyo3::types::PyList::new(py, &py_items)
            .map_err(|e| anyhow::anyhow!("Failed to create list: {e}"))?;

        let first = items
            .first()
            .ok_or_else(|| anyhow::anyhow!("No items to encode"))?;
        let first_py = first.to_pyobject(py)?;

        if first_py
            .bind(py)
            .hasattr("encode_record_batch_py")
            .unwrap_or(false)
        {
            let py_batch = first_py
                .bind(py)
                .call_method1("encode_record_batch_py", (py_list,))
                .map_err(|e| anyhow::anyhow!("Failed to call encode_record_batch_py: {e}"))?;

            let mut ffi_array = arrow::ffi::FFI_ArrowArray::empty();
            let mut ffi_schema = arrow::ffi::FFI_ArrowSchema::empty();

            py_batch.call_method1(
                "_export_to_c",
                (

View on GitHub (pinned to 18893faf8b)

Solutions

Guard the call site: skip encoding (or produce an empty batch another way) when the collection is empty.
Check `len(items)` before invoking the encode path for custom data.
Fix upstream logic that produces empty batches and ensure they are filtered before persistence.

Example fix

# before
batch = encode_custom_data(items)  # raises when items == []

# after
batch = encode_custom_data(items) if items else None
if batch is not None:
    write(batch)

Defensive patterns

Strategy: validation

Validate before calling

if not items:
    return None  # skip encoding empty custom data batches

Try / catch

if items:
    batch = encode_to_record_batch(items)
else:
    batch = None

Prevention

Filter empty collections before calling any encode path
Handle the empty-batch case explicitly in persistence pipelines
Never rely on the encoder to tolerate zero items

When it happens

Trigger: Calling the custom-data Arrow encode path (via registered custom data classes) with an empty `items` slice — e.g. encoding an empty list of custom data objects to a RecordBatch.

Common situations: A data pipeline forwards an empty collection (no custom data received yet) straight to the catalog writer without checking emptiness; batching logic produces an empty final batch that is still sent to encode.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/e456cc030380888b. Report an issue: GitHub.