pola-rs/polars · critical

not implemented

Error message

not implemented

What it means

Panic (unimplemented!()) in polars-arrow's make_builder, which constructs an ArrayBuilder from an ArrowDataType. The physical-type match covers Null, Boolean, Primitive, LargeBinary, FixedSizeBinary, LargeList, FixedSizeList, Struct, BinaryView and Utf8View; requesting a builder for 32-bit List, Binary/Utf8/LargeUtf8 (non-view strings), Map, Union, or Dictionary dtypes panics with 'not implemented'. Polars internally standardizes on the view/large variants, so these dtypes reaching this function are unhandled by design.

Source

Thrown at crates/polars-arrow/src/array/builder.rs:379

                unreachable!()
            };
            Box::new(FixedSizeListArrayBuilder::new(
                dtype.clone(),
                make_builder(inner_dt.dtype()),
            ))
        },
        Struct => {
            let ArrowDataType::Struct(fields) = dtype else {
                unreachable!()
            };
            let builders = fields.iter().map(|f| make_builder(f.dtype())).collect();
            Box::new(StructArrayBuilder::new(dtype.clone(), builders))
        },
        BinaryView => Box::new(BinaryViewArrayGenericBuilder::<[u8]>::new(dtype.clone())),
        Utf8View => Box::new(BinaryViewArrayGenericBuilder::<str>::new(dtype.clone())),

        List | Binary | Utf8 | LargeUtf8 | Map | Union | Dictionary(_) => {
            unimplemented!()
        },
    }
}

View on GitHub (pinned to df599052da)

Solutions

1. Convert legacy dtypes before touching polars-arrow builders: Utf8/LargeUtf8 -> Utf8View (or LargeUtf8 where supported), Binary -> LargeBinary, List -> LargeList, Dictionary -> plain values or Categorical via polars-core.
2. Rechunk/convert in pyarrow first: `tbl.cast(target_schema)` with view/large types, or pass through polars' own interchange (pl.from_arrow) which normalizes dtypes.
3. If you control the producer, write Arrow IPC with view/large types (polars' default).
4. Upgrade polars - builder coverage grows across releases; if the panic persists, report the dtype combination upstream.
5. As a library author, avoid calling make_builder (or CreateBuilder) on unvalidated schemas; validate against the supported set first.

Example fix

// before (Rust)
let b = polars_arrow::array::builder::make_builder(&ArrowDataType::Utf8); // panics

// after
let dtype = ArrowDataType::Utf8View;
let b = polars_arrow::array::builder::make_builder(&dtype); // BinaryViewArrayGenericBuilder<str>

// Rust
use polars_arrow::datatypes::ArrowDataType;
fn builder_supported(dt: &ArrowDataType) -> bool {
    use ArrowDataType::*;
    !matches!(dt, List(_) | Binary | Utf8 | LargeUtf8 | Map(_, _) | Union(_, _) | Dictionary(_, _))
}
assert!(builder_supported(dt), "unsupported builder dtype: {dt:?}");

// Rust
fn is_supported_builder_dtype(dt: &ArrowDataType) -> bool {
    !matches!(dt, ArrowDataType::List(_) | ArrowDataType::Binary | ArrowDataType::Utf8 | ArrowDataType::LargeUtf8 | ArrowDataType::Map(_, _) | ArrowDataType::Union(_, _) | ArrowDataType::Dictionary(_, _))
}

// Rust - last resort containment for a panic boundary
let result = std::panic::catch_unwind(|| make_builder(&dt));
if result.is_err() {
    // normalize dtype (Utf8->Utf8View, List->LargeList) and retry, or surface an error
}

When it happens

Trigger: Any polars-arrow path that creates builders from an externally supplied Arrow schema containing e.g. ArrowDataType::Utf8, ArrowDataType::List, ArrowDataType::Dictionary(Int32, Utf8), or Map - e.g. concatenation/extend paths (CreateBuilder/make_builder recursion into LargeList inner dtypes also propagates the panic), or Series/DataFrame construction from arrow-rs record batches with those legacy dtypes via the Arrow C data interface.

Common situations: Ingesting Arrow IPC/Feather files or arrow-rs RecordBatches whose strings are Utf8 instead of Utf8View, or whose lists are 32-bit List; dictionaries/categoricals from other Arrow systems; interop layers (pyarrow <-> polars) on older polars versions; nesting (LargeList of Dictionary) hitting the inner make_builder call.

Related errors

• not implemented
• horizontal_flatten not supported for data type {:?}
• can not get dtype of Categorical AnyValue
• can not get dtype of Enum AnyValue
• Deserialization from JSON not implemented for {adt:?}

AI-assisted analysis of pola-rs/polars@df599052da (2026-08-16). Data as JSON: /api/errors/4aadc11e2653acf5. Report an issue: GitHub.