What causes "can not get dtype of Enum AnyValue" in pola-rs/polars?

Read the dtype from the Series or schema rather than from an individual value

can not get dtype of Enum AnyValue

Error message

can not get dtype of Enum AnyValue

What it means

AnyValue::dtype() panics for Enum/EnumOwned scalars. As with categoricals, an enum scalar holds only the category index and a reference to the frozen categories, so a faithful DataType::Enum cannot be produced cheaply and the arm is unimplemented.

Source

Thrown at crates/polars-core/src/datatypes/any_value.rs:271

            Binary(_) | BinaryOwned(_) => DataType::Binary,
            #[cfg(feature = "dtype-date")]
            Date(_) => DataType::Date,
            #[cfg(feature = "dtype-time")]
            Time(_) => DataType::Time,
            #[cfg(feature = "dtype-datetime")]
            Datetime(_, tu, tz) => DataType::Datetime(*tu, (*tz).cloned()),
            #[cfg(feature = "dtype-datetime")]
            DatetimeOwned(_, tu, tz) => {
                DataType::Datetime(*tu, tz.as_ref().map(|v| v.as_ref().clone()))
            },
            #[cfg(feature = "dtype-duration")]
            Duration(_, tu) => DataType::Duration(*tu),
            #[cfg(feature = "dtype-categorical")]
            Categorical(_, _) | CategoricalOwned(_, _) => {
                unimplemented!("can not get dtype of Categorical AnyValue")
            },
            #[cfg(feature = "dtype-categorical")]
            Enum(_, _) | EnumOwned(_, _) => unimplemented!("can not get dtype of Enum AnyValue"),
            List(s) => DataType::List(Box::new(s.dtype().clone())),
            #[cfg(feature = "dtype-array")]
            Array(s, size) => DataType::Array(Box::new(s.dtype().clone()), *size),
            #[cfg(feature = "dtype-struct")]
            Struct(_, _, fields) => DataType::Struct(fields.to_vec()),
            #[cfg(feature = "dtype-struct")]
            StructOwned(payload) => DataType::Struct(payload.1.clone()),
            #[cfg(feature = "dtype-decimal")]
            Decimal(_, p, s) => DataType::Decimal(*p, *s),
            #[cfg(feature = "object")]
            Object(o) => DataType::Object(o.type_name()),
            #[cfg(feature = "object")]
            ObjectOwned(o) => DataType::Object(o.0.type_name()),
        }
    }

    /// Extract a numerical value from the AnyValue
    #[doc(hidden)]

View on GitHub (pinned to df599052da)

Solutions

Read the dtype from the Series or schema rather than from an individual value
Convert the scalar to String when only the value is needed
Handle the Enum(_, _) AnyValue variant explicitly in Rust code that must inspect it

Example fix

# before
av = df["grade"].get(0)  # grade is pl.Enum
av.dtype()  # panics

# after
df["grade"].dtype  # Enum dtype from the column
av.cast(pl.String)  # value-only use

Defensive patterns

Strategy: validation

Validate before calling

if isinstance(df["grade"].dtype, pl.Enum):
    dtype = df["grade"].dtype
else:
    dtype = df["grade"].get(0).dtype()

Type guard

def scalar_dtype_safe(series: pl.Series, idx: int = 0):
    if isinstance(series.dtype, (pl.Categorical, pl.Enum)):
        return series.dtype
    return series.get(idx).dtype()

Prevention

Keep enum dtype information in the schema, not in extracted scalars
Treat pl.Enum columns specially in generic scalar-introspection code

When it happens

Trigger: Calling .dtype() on an AnyValue obtained from an Enum column (pl.Enum([...])): df["grade"].get(0).dtype(), or library code that maps scalars back to dtypes (e.g. inferring a schema from Python-side values).

Common situations: Schema-from-scalars utilities, generic scalar introspection after switching a column to pl.Enum for memory or correctness reasons.

Related errors

AI-assisted analysis of pola-rs/polars@df599052da (2026-08-16). Data as JSON: /api/errors/73c60a67a709a81f. Report an issue: GitHub.