polars.Expr.dt.month_start#

Expr.dt.month_start() → Expr[source]#

Roll backward to the first day of the month.

For datetimes, the time-of-day is preserved.

Returns:

Expr: Expression of data type Date or Datetime.

Notes

If you’re coming from pandas, you can think of this as a vectorised version of pandas.tseries.offsets.MonthBegin().rollback(datetime).

Examples

>>> from datetime import datetime
>>> df = pl.DataFrame(
...     {
...         "dates": pl.datetime_range(
...             datetime(2000, 1, 15, 2),
...             datetime(2000, 12, 15, 2),
...             "1mo",
...             eager=True,
...         )
...     }
... )
>>> df.select(pl.col("dates").dt.month_start())
shape: (12, 1)
┌─────────────────────┐
│ dates               │
│ ---                 │
│ datetime[μs]        │
╞═════════════════════╡
│ 2000-01-01 02:00:00 │
│ 2000-02-01 02:00:00 │
│ 2000-03-01 02:00:00 │
│ 2000-04-01 02:00:00 │
│ 2000-05-01 02:00:00 │
│ …                   │
│ 2000-08-01 02:00:00 │
│ 2000-09-01 02:00:00 │
│ 2000-10-01 02:00:00 │
│ 2000-11-01 02:00:00 │
│ 2000-12-01 02:00:00 │
└─────────────────────┘