pola-rs/polars · error

to_list not implemented for unknown

Error message

to_list not implemented for unknown

What it means

Guard in Series.to_list (python export): the series dtype is Unknown (e.g. from an unresolved lazy schema or dsl scan), for which no Python list conversion exists. Resolve the schema (collect with known dtypes) before exporting.

Source

Thrown at crates/polars-python/src/series/export.rs:161

                        iter: RepeatN<Option<u8>>,
                        n: usize,
                    }
                    impl Iterator for NullIter {
                        type Item = Option<u8>;

                        fn next(&mut self) -> Option<Self::Item> {
                            self.iter.next()
                        }
                        fn size_hint(&self) -> (usize, Option<usize>) {
                            (self.n, Some(self.n))
                        }
                    }
                    impl ExactSizeIterator for NullIter {}

                    PyList::new(py, NullIter { iter, n })?
                },
                DataType::Unknown(_) => {
                    panic!("to_list not implemented for unknown")
                },
                DataType::BinaryOffset => {
                    unreachable!()
                },
                DataType::Map(_, _) => {
                    let ca = series.map().map_err(PyPolarsErr::from)?;
                    PyList::new(py, ca.any_value_iter().map(Wrap))?
                },
                DataType::Extension(_, _) => {
                    return to_list_recursive(py, series.ext().unwrap().storage());
                },
            };
            Ok(pylist.into_any())
        }

        to_list_recursive(py, series)
    }

View on GitHub (pinned to 68506541d2)

Solutions

1. Cast the Series to a concrete dtype before calling to_list(); 'unknown' columns come from empty/all-null data — give them a dtype via cast or schema.

Example fix

s = pl.Series('a', [], dtype=pl.Int64)
s.to_list()

When it happens

Trigger: Calling Series.to_list() on a Series of dtype Unknown.

Common situations: See trigger scenarios.

AI-assisted analysis of pola-rs/polars@68506541d2 (2026-08-19). Data as JSON: /api/errors/cb61066a40b480be. Report an issue: GitHub.