pola-rs/polars · error · TypeError

invalid dtype for `ones`; found {dtype}

Error message

invalid dtype for `ones`; found {dtype}

What it means

Raised by pl.ones when the requested dtype has no sensible 'one' representation. The helper _one_or_zero_by_dtype only supports integer dtypes, float dtypes, Boolean, String (Utf8), Decimal, and List/Array of those; anything else returns None and triggers this TypeError.

Source

Thrown at py-polars/src/polars/functions/repeat.py:223

    See Also
    --------
    repeat
    lit

    Examples
    --------
    >>> pl.ones(3, pl.Int8, eager=True)
    shape: (3,)
    Series: 'ones' [i8]
    [
        1
        1
        1
    ]
    """
    if (one := _one_or_zero_by_dtype(1, dtype)) is None:
        msg = f"invalid dtype for `ones`; found {dtype}"
        raise TypeError(msg)

    return repeat(one, n=n, dtype=dtype, eager=eager).alias("ones")


@overload
def zeros(
    n: int | Expr,
    dtype: PolarsDataType = ...,
    *,
    eager: Literal[False] = ...,
) -> Expr: ...


@overload
def zeros(
    n: int | Expr,
    dtype: PolarsDataType = ...,
    *,

View on GitHub (pinned to df599052da)

Solutions

1. Use a supported dtype (Int/Float/Boolean/String/Decimal or List/Array of those) then .cast() to the target dtype: pl.ones(3, pl.Int64).cast(pl.Date)
2. For arbitrary literal fills, use pl.repeat(value, n, dtype=...) which accepts a broader value/dtype combination
3. For datetimes, build from ones on integers: (pl.ones(3, pl.Int64) * 0).cast(Datatype) or pl.repeat + cast

Example fix

// before
pl.ones(3, pl.Date, eager=True)
// after
pl.ones(3, pl.Int64, eager=True).cast(pl.Date)

from polars.datatypes.group import FLOAT_DTYPES, INTEGER_DTYPES
from polars import Boolean, Utf8, Decimal, List, Array

def ones_supported(dtype) -> bool:
    return (dtype in INTEGER_DTYPES or dtype in FLOAT_DTYPES
            or dtype in (Boolean, Utf8) or isinstance(dtype, (Decimal, List, Array)))

def is_fillable_dtype(dtype) -> bool:
    from polars.datatypes.group import INTEGER_DTYPES, FLOAT_DTYPES
    from polars import Boolean, Utf8, Decimal, List, Array
    return (dtype in INTEGER_DTYPES or dtype in FLOAT_DTYPES
            or dtype in (Boolean, Utf8) or isinstance(dtype, (Decimal, List, Array)))

try:
    s = pl.ones(3, dtype, eager=True)
except TypeError:
    s = pl.ones(3, pl.Int64, eager=True).cast(dtype)

When it happens

Trigger: pl.ones(3, pl.Date); pl.ones(3, dtype=Datetime('us')); pl.ones(3, pl.Categorical); pl.ones(3, pl.Duration('ms')); nested List of an unsupported inner dtype.

Common situations: Trying to preallocate typed dummy columns for schemas that include temporal/categorical dtypes; porting numpy.ones-style initialization to polars; assuming any dtype works for a constant fill.

Related errors

• invalid dtype for `zeros`; found {dtype}
• cannot select columns using key of type {qualified_type_name
• expected {df.width} values when selecting columns by boolean
• index {key} is out of bounds for DataFrame of height {num_ro
• cannot select rows using key of type {qualified_type_name(ke

AI-assisted analysis of pola-rs/polars@df599052da (2026-08-16). Data as JSON: /api/errors/6427b3c2a9ad02b1. Report an issue: GitHub.