pola-rs/polars · error

Expecting to be able to downcast into bytes from read result

Error message

Expecting to be able to downcast into bytes from read result.

What it means

Internal expect() in PyFileLikeObject::to_buffer: the object returned by the Python file's read() is neither bytes nor str, so it cannot be downcast into the byte payload Polars needs. The file-like object returned an unexpected type from read.

Source

Thrown at crates/polars-python/src/file.rs:106

                let owner = bytes.clone_ref(py);
                let ss = unsafe { SharedStorage::from_slice_with_owner(slice, owner) };
                return Buffer::from_storage(ss);
            }

            if let Ok(b) = bytes.cast_bound::<PyString>(py) {
                return match b.to_cow().expect("PyString is not valid UTF-8") {
                    Cow::Borrowed(v) => {
                        // SAFETY: we keep the underlying python object alive.
                        let slice = v.as_bytes();
                        let owner = bytes.clone_ref(py);
                        let ss = unsafe { SharedStorage::from_slice_with_owner(slice, owner) };
                        return Buffer::from_storage(ss);
                    },
                    Cow::Owned(v) => Buffer::from_vec(v.into_bytes()),
                };
            }

            panic!("Expecting to be able to downcast into bytes from read result.");
        })
    }

    /// Validates that the underlying
    /// python object has a `read`, `write`, and `seek` methods in respect to parameters.
    /// Will return a `TypeError` if object does not have `read`, `seek`, and `write` methods.
    pub(crate) fn ensure_requirements(
        object: &Bound<PyAny>,
        read: bool,
        write: bool,
        seek: bool,
    ) -> PyResult<()> {
        if read && object.getattr("read").is_err() {
            return Err(PyErr::new::<PyTypeError, _>(
                "Object does not have a .read() method.",
            ));
        }

View on GitHub (pinned to 5d8ebabf11)

Solutions

1. The file-like object's read() must return bytes; wrap text readers to return bytes.
2. Open the file in binary mode ('rb') before passing it.

When it happens

Trigger: This panic/expect fires when execution reaches an unguarded state described by: "Expecting to be able to downcast into bytes from read result.". Typical triggers: unsupported dtype or feature-gated code path reached at runtime, invalid user input or environment variable value, calling API methods in the wrong order or on mismatched types, or data (lengths, offsets, ranges) violating the function's preconditions.

Common situations: Encountered when input data or configuration does not meet the preconditions of the throwing code path ("Expecting to be able to downcast into bytes from read result."). Common cases: missing Cargo feature flags, mismatched dtypes/lengths between arrays or series, out-of-range temporal values, malformed environment variables, or operations on unsupported/complex nested types.

AI-assisted analysis of pola-rs/polars@5d8ebabf11 (2026-08-19). Data as JSON: /api/errors/dfd4d4d480ed0279. Report an issue: GitHub.