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narwhals.selectors

The following selectors are all supported. In addition, just like in Polars, the following set operations are supported:

  • set intersection: &
  • set union: |
  • set difference: -
  • complement: ~

boolean 

boolean() -> Selector

Select boolean columns.

Returns:

Type Description
Selector

A new expression.

Examples:

>>> import polars as pl
>>> import narwhals as nw
>>> import narwhals.selectors as ncs
>>> df_native = pl.DataFrame({"a": [1, 2], "b": ["x", "y"], "c": [False, True]})
>>> df = nw.from_native(df_native)

Let's select boolean dtypes:

>>> df.select(ncs.boolean())
┌──────────────────┐
|Narwhals DataFrame|
|------------------|
|  shape: (2, 1)   |
|  ┌───────┐       |
|  │ c     │       |
|  │ ---   │       |
|  │ bool  │       |
|  ╞═══════╡       |
|  │ false │       |
|  │ true  │       |
|  └───────┘       |
└──────────────────┘

by_dtype 

by_dtype(
    *dtypes: DType
    | type[DType]
    | Iterable[DType | type[DType]],
) -> Selector

Select columns based on their dtype.

Parameters:

Name Type Description Default
dtypes DType | type[DType] | Iterable[DType | type[DType]]

one or data types to select

 ()

Returns:

Type Description
Selector

A new expression.

Examples:

>>> import pyarrow as pa
>>> import narwhals as nw
>>> import narwhals.selectors as ncs
>>> df_native = pa.table({"a": [1, 2], "b": ["x", "y"], "c": [4.1, 2.3]})
>>> df = nw.from_native(df_native)

Let's select int64 and float64 dtypes and multiply each value by 2:

>>> df.select(ncs.by_dtype(nw.Int64, nw.Float64) * 2).to_native()
pyarrow.Table
a: int64
c: double
----
a: [[2,4]]
c: [[8.2,4.6]]

categorical 

categorical() -> Selector

Select categorical columns.

Returns:

Type Description
Selector

A new expression.

Examples:

>>> import polars as pl
>>> import narwhals as nw
>>> import narwhals.selectors as ncs
>>> df_native = pl.DataFrame({"a": [1, 2], "b": ["x", "y"], "c": [False, True]})

Let's convert column "b" to categorical, and then select categorical dtypes:

>>> df = nw.from_native(df_native).with_columns(
...     b=nw.col("b").cast(nw.Categorical())
... )
>>> df.select(ncs.categorical()).to_native()
shape: (2, 1)
┌─────┐
│ b   │
│ --- │
│ cat │
╞═════╡
│ x   │
│ y   │
└─────┘

datetime 

datetime(
    time_unit: TimeUnit | Iterable[TimeUnit] | None = None,
    time_zone: (
        str
        | timezone
        | Iterable[str | timezone | None]
        | None
    ) = ("*", None),
) -> Selector

Select all datetime columns, optionally filtering by time unit/zone.

Parameters:

Name Type Description Default
time_unit TimeUnit | Iterable[TimeUnit] | None

One (or more) of the allowed timeunit precision strings, "ms", "us", "ns" and "s". Omit to select columns with any valid timeunit.

 None
time_zone str | timezone | Iterable[str | timezone | None] | None

Specify which timezone(s) to select:

  • One or more timezone strings, as defined in zoneinfo (to see valid options run import zoneinfo; zoneinfo.available_timezones() for a full list).
  • Set None to select Datetime columns that do not have a timezone.
  • Set "*" to select Datetime columns that have any timezone.
 ('*', None)

Returns:

Type Description
Selector

A new expression.

Examples:

>>> from datetime import datetime, timezone
>>> import pyarrow as pa
>>> import narwhals as nw
>>> import narwhals.selectors as ncs
>>>
>>> utc_tz = timezone.utc
>>> data = {
...     "tstamp_utc": [
...         datetime(2023, 4, 10, 12, 14, 16, 999000, tzinfo=utc_tz),
...         datetime(2025, 8, 25, 14, 18, 22, 666000, tzinfo=utc_tz),
...     ],
...     "tstamp": [
...         datetime(2000, 11, 20, 18, 12, 16, 600000),
...         datetime(2020, 10, 30, 10, 20, 25, 123000),
...     ],
...     "numeric": [3.14, 6.28],
... }
>>> df_native = pa.table(data)
>>> df_nw = nw.from_native(df_native)
>>> df_nw.select(ncs.datetime()).to_native()
pyarrow.Table
tstamp_utc: timestamp[us, tz=UTC]
tstamp: timestamp[us]
----
tstamp_utc: [[2023-04-10 12:14:16.999000Z,2025-08-25 14:18:22.666000Z]]
tstamp: [[2000-11-20 18:12:16.600000,2020-10-30 10:20:25.123000]]

Select only datetime columns that have any time_zone specification:

>>> df_nw.select(ncs.datetime(time_zone="*")).to_native()
pyarrow.Table
tstamp_utc: timestamp[us, tz=UTC]
----
tstamp_utc: [[2023-04-10 12:14:16.999000Z,2025-08-25 14:18:22.666000Z]]

matches 

matches(pattern: str) -> Selector

Select all columns that match the given regex pattern.

Parameters:

Name Type Description Default
pattern str

A valid regular expression pattern.

 required

Returns:

Type Description
Selector

A new expression.

Examples:

>>> import pandas as pd
>>> import narwhals as nw
>>> import narwhals.selectors as ncs
>>> df_native = pd.DataFrame(
...     {
...         "bar": [123, 456],
...         "baz": [2.0, 5.5],
...         "zap": [0, 1],
...     }
... )
>>> df = nw.from_native(df_native)

Let's select column names containing an 'a', preceded by a character that is not 'z':

>>> df.select(ncs.matches("[^z]a")).to_native()
   bar  baz
0  123  2.0
1  456  5.5

numeric 

numeric() -> Selector

Select numeric columns.

Returns:

Type Description
Selector

A new expression.

Examples:

>>> import polars as pl
>>> import narwhals as nw
>>> import narwhals.selectors as ncs
>>> df_native = pl.DataFrame({"a": [1, 2], "b": ["x", "y"], "c": [4.1, 2.3]})
>>> df = nw.from_native(df_native)

Let's select numeric dtypes and multiply each value by 2:

>>> df.select(ncs.numeric() * 2).to_native()
shape: (2, 2)
┌─────┬─────┐
│ a   ┆ c   │
│ --- ┆ --- │
│ i64 ┆ f64 │
╞═════╪═════╡
│ 2   ┆ 8.2 │
│ 4   ┆ 4.6 │
└─────┴─────┘

string 

string() -> Selector

Select string columns.

Returns:

Type Description
Selector

A new expression.

Examples:

>>> import polars as pl
>>> import narwhals as nw
>>> import narwhals.selectors as ncs
>>> df_native = pl.DataFrame({"a": [1, 2], "b": ["x", "y"], "c": [False, True]})
>>> df = nw.from_native(df_native)

Let's select string dtypes:

>>> df.select(ncs.string()).to_native()
shape: (2, 1)
┌─────┐
│ b   │
│ --- │
│ str │
╞═════╡
│ x   │
│ y   │
└─────┘