Polars: Create a new column with an array of dates from two different date columns storing the start and end of the range

Question

I have a dataframe with a date start and date end (Start Promotion and End Promotion). I would like to use Python Polars to create a new column (from the previously mentioned columns) that will be an array or struct object. The purpose in this is so that the column can be exploded creating a value for each date that can be joined to another data frame later.

This is an example data frame:

Start Promotion	End Promotion	Price
2024-05-01	2024-05-05	12
2023-12-30	2024-01-01	10

The desired end result is:

Start Promotion	End Promotion	Price	Promotion Dates
2024-05-01	2024-05-05	12	["2024-05-01", "2024-05-02", "2024-05-03", "2024-05-04", "2024-05-05"]
2023-12-30	2024-01-01	10	["2023-12-30", "2023-12-31", "2024-01-01"]

Here is an example of what it should look like if exploded:

Start Promotion	End Promotion	Price	Promotion Dates
2024-05-01	2024-05-05	12	"2024-05-01"
2024-05-01	2024-05-05	12	"2024-05-02"
2024-05-01	2024-05-05	12	"2024-05-03"
2024-05-01	2024-05-05	12	"2024-05-04"
2024-05-01	2024-05-05	12	"2024-05-05"
2023-12-30	2024-01-01	10	"2023-12-30"
2023-12-30	2024-01-01	10	"2023-12-31"
2023-12-30	2024-01-01	10	"2024-01-01"

Here is an example data frame to attempt it with:

import polars as pl

data = {
    "Start Promotion": ["2024-05-01", "2023-12-30"],
    "End Promotion": ["2024-05-05", "2024-01-01"],
    "Price": [12, 10]
}

df = pl.DataFrame(data)

Let me know if you have any clarifying questions.

roman · Accepted Answer · 2024-06-01 06:42:34Z

you can use .date_ranges() for that:

(
    df
    .with_columns(
        pl.date_ranges("Start Promotion", "End Promotion").alias("Promotion Dates")
    )
)

┌─────────────────┬───────────────┬───────┬─────────────────────────────────┐
│ Start Promotion ┆ End Promotion ┆ Price ┆ Promotion Dates                 │
│ ---             ┆ ---           ┆ ---   ┆ ---                             │
│ date            ┆ date          ┆ i64   ┆ list[date]                      │
╞═════════════════╪═══════════════╪═══════╪═════════════════════════════════╡
│ 2024-05-01      ┆ 2024-05-05    ┆ 12    ┆ [2024-05-01, 2024-05-02, … 202… │
│ 2023-12-30      ┆ 2024-01-01    ┆ 10    ┆ [2023-12-30, 2023-12-31, 2024-… │
└─────────────────┴───────────────┴───────┴─────────────────────────────────┘

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