pola-rs/polars · error · TypeError

cannot convert List dtype to Tensor (use Array dtype instead

Error message

cannot convert List dtype to Tensor (use Array dtype instead)

What it means

to_torch conversion guard: PyTorch tensors need fixed-shape elements, so a List-dtype Series cannot be converted; the guard directs users to the fixed-width Array dtype. Raised before any tensor construction.

Source

Thrown at py-polars/src/polars/series/series.py:4985

        torch = import_optional("torch")

        # PyTorch tensors do not support uint16/32/64
        if self.dtype in (UInt32, UInt64):
            srs = self.cast(Int64)
        elif self.dtype == UInt16:
            srs = self.cast(Int32)
        else:
            srs = self

        # we have to build the tensor from a writable array or PyTorch will complain
        # about it (writing to a readonly array results in undefined behavior)
        numpy_array = srs.to_numpy(writable=True)
        try:
            tensor = torch.from_numpy(numpy_array)
        except TypeError:
            if self.dtype == List:
                msg = "cannot convert List dtype to Tensor (use Array dtype instead)"
                raise TypeError(msg) from None
            raise
        # note: named tensors are currently experimental
        # tensor.rename(self.name)
        return tensor

    @removed_parameters(
        RenamedParameter(
            name="future",
            new_name="compat_level",
            deprecated_in="1.1",
            removed_in="2.0",
        ),
    )
    def to_arrow(self, *, compat_level: CompatLevel | None = None) -> pa.Array:
        """
        Return the underlying Arrow array.

        If the Series contains only a single chunk this operation is zero copy.

View on GitHub (pinned to 68506541d2)

Solutions

1. Convert the column to the fixed-width Array dtype (e.g. `cast(pl.Array(inner, size))`) before converting to a tensor.

When it happens

Trigger: Thrown at py-polars/src/polars/series/series.py:5145 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of pola-rs/polars@68506541d2 (2026-08-19). Data as JSON: /api/errors/0d284de271bddf1b. Report an issue: GitHub.