pola-rs/polars · error · TypeError

Enum categories must be strings; found data of type {dtype}

Error message

Enum categories must be strings; found data of type {dtype}

What it means

Error "Enum categories must be strings; found data of type {dtype}" thrown in pola-rs/polars.

Source

Thrown at py-polars/src/polars/datatypes/classes.py:1065

            enum_values = [
                getattr(v, "value", v) for v in categories.__members__.values()
            ]
            categories = pl.Series(values=enum_values)
        elif not isinstance(categories, pl.Series):
            categories = pl.Series(values=categories)

        if categories.is_empty():
            self.categories = pl.Series(name="category", dtype=String)
            return

        if categories.has_nulls():
            msg = "Enum categories must not contain null values"
            raise TypeError(msg)

        if (dtype := categories.dtype) != String:
            msg = f"Enum categories must be strings; found data of type {dtype}"
            raise TypeError(msg)

        if categories.n_unique() != categories.len():
            duplicate = categories.filter(categories.is_duplicated())[0]
            msg = f"Enum categories must be unique; found duplicate {duplicate!r}"
            raise ValueError(msg)

        self.categories = categories.rechunk().alias("category")

    def __eq__(self, other: PolarsDataType) -> bool:  # type: ignore[override]
        # allow comparing object instances to class
        if type(other) is DataTypeClass and issubclass(other, Enum):
            return True
        elif isinstance(other, Enum):
            return self.categories.equals(other.categories)
        else:
            return False

    def __hash__(self) -> int:

View on GitHub (pinned to df599052da)

When it happens

Trigger: Thrown at py-polars/src/polars/datatypes/classes.py:1065 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of pola-rs/polars@df599052da (2026-08-16). Data as JSON: /api/errors/2bbb3ae0c301e398. Report an issue: GitHub.