CODEWITHSUNDEEP
pythondatetimepython-polars
seb2704
seb2704

Reputation: 540

cast datetime[ms] to datetime[ns] in polars

I'd like to cast a column with the type datetype[ms] to datetime[ns] in polars. Is there a easy way to do this?

I think there's a problem with group_by_dynamic when using datetime[ms] and I'd like to test this

Upvotes: 3

Views: 6374

Answers (2)

ritchie46
ritchie46

Reputation: 14670

You can use .dt.cast_time_unit()

s = pl.datetime_range(
        pl.datetime(2021, 1, 1), pl.datetime(2021, 1, 2),
        interval="3h", time_unit="ns", eager=True
)

s.dt.cast_time_unit("ms")

shape: (9,)
Series: '' [datetime[ms]]
[
    2021-01-01 00:00:00
    2021-01-01 03:00:00
    2021-01-01 06:00:00
    2021-01-01 09:00:00
    2021-01-01 12:00:00
    2021-01-01 15:00:00
    2021-01-01 18:00:00
    2021-01-01 21:00:00
    2021-01-02 00:00:00
]

Upvotes: 9

Banana
Banana

Reputation: 1187

You can also use cast_time_unit directly. It works more generally to convert datetime[x] to datetime[y] without you requiring to think about conversion factors. For example:

df = pl.DataFrame(data = {
    "date": pl.datetime_range(
        pl.datetime(2021, 1, 1), pl.datetime(2021, 1, 2),
        interval="3h", time_unit="ns", eager=True,
    )}
)

df = df.with_columns(pl.col("date").dt.cast_time_unit("ms"))

Also produces the desired result

>>> print(df)
shape: (9, 1)
┌─────────────────────┐
│ date                │
│ ---                 │
│ datetime[ms]        │
╞═════════════════════╡
│ 2021-01-01 00:00:00 │
│ 2021-01-01 03:00:00 │
│ 2021-01-01 06:00:00 │
│ 2021-01-01 09:00:00 │
│ 2021-01-01 12:00:00 │
│ 2021-01-01 15:00:00 │
│ 2021-01-01 18:00:00 │
│ 2021-01-01 21:00:00 │
│ 2021-01-02 00:00:00 │
└─────────────────────┘

Upvotes: 2

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