How to roll up duplicate observation in Python polars?

I have a data frame as-

my_dt = pl.DataFrame({'last_name':['mallesh','bhavik','jagarini','mallesh','jagarini'],
                  'first_name':['yamulla','vemulla','yegurla','yamulla','yegurla'],
                  'ssn':['1234','7847','0648','4567','0648']})

Here I would like to find out duplicates considering last_name and firs_name columns and if any duplicates found their respective ssn needs to be rolled up with semicolon(;) if SSN are not different. if SSN are also same only one SSN needs to be present.

the expected output as:

Here since mallesh yamulla is duplicated and has different SSN’s they are rolled up with ‘;’

and in case of jagarini yegurla it has a unique SSN hence one SSN is only taken.

Added one more case:

Here on given any set of column it should rollup the unique values using ; from the remaining columns. here on last and first name, roll up should be done on both DOB and SSN.

my_dt = pl.DataFrame({'last_name':['mallesh','bhavik','jagarini','mallesh','jagarini'],
                  'first_name':['yamulla','vemulla','yegurla','yamulla','yegurla'],
                  'ssn':['1234','7847','0648','4567','0648'],
                  'dob':['10/11/1990','09/16/1991','01/01/1990','10/11/1990','02/14/1983']   })

Another case as:

my_dt = pl.DataFrame({'last_name':['mallesh','bhavik','jagarini','mallesh','jagarini'],
                  'first_name':['yamulla','vemulla','yegurla','yamulla','yegurla'],
                  'ssn':['1234','7847','0648','4567','0648'],
                  'dob':['10/11/1990','09/16/1991','01/01/1990','','02/14/1983']   })

In case of having null values in a field it should treat as empty not as a value.

“;10/11/1990” it should just be “10/11/1990” for mallesh yamulla entry.

Answer

Use a group_by and unique to remove duplicates. From there, you can use arr.join on the resulting list.

(
    my_dt
    .groupby(['last_name', 'first_name'])
    .agg([
        pl.col('ssn').unique()
    ])
    .with_column(
        pl.col('ssn').arr.join(';')
    )
)

shape: (3, 3)
┌───────────┬────────────┬───────────┐
│ last_name ┆ first_name ┆ ssn       │
│ ---       ┆ ---        ┆ ---       │
│ str       ┆ str        ┆ str       │
╞═══════════╪════════════╪═══════════╡
│ mallesh   ┆ yamulla    ┆ 4567;1234 │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌┤
│ bhavik    ┆ vemulla    ┆ 7847      │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌┤
│ jagarini  ┆ yegurla    ┆ 0648      │
└───────────┴────────────┴───────────┘

Edit: if you want to ensure that the rolled up list is sorted:

(
    my_dt
    .groupby(['last_name', 'first_name'])
    .agg([
        pl.col('ssn')
        .unique()
        .sort()
    ])
    .with_column(
        pl.col('ssn')
        .arr.join(';')
    )
)

shape: (3, 3)
┌───────────┬────────────┬───────────┐
│ last_name ┆ first_name ┆ ssn       │
│ ---       ┆ ---        ┆ ---       │
│ str       ┆ str        ┆ str       │
╞═══════════╪════════════╪═══════════╡
│ jagarini  ┆ yegurla    ┆ 0648      │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌┤
│ mallesh   ┆ yamulla    ┆ 1234;4567 │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌┤
│ bhavik    ┆ vemulla    ┆ 7847      │
└───────────┴────────────┴───────────┘

Edit: Rolling up multiple columns

We can roll up multiple columns elegantly as follows:

(
    my_dt
    .groupby(["last_name", "first_name"])
    .agg([
        pl.all().unique().sort().cast(pl.Utf8)
    ])
    .with_columns([
        pl.exclude(['last_name', 'first_name']).arr.join(";")
    ])
)

shape: (3, 4)
┌───────────┬────────────┬───────────┬───────────────────────┐
│ last_name ┆ first_name ┆ ssn       ┆ dob                   │
│ ---       ┆ ---        ┆ ---       ┆ ---                   │
│ str       ┆ str        ┆ str       ┆ str                   │
╞═══════════╪════════════╪═══════════╪═══════════════════════╡
│ bhavik    ┆ vemulla    ┆ 7847      ┆ 1991-09-16            │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ jagarini  ┆ yegurla    ┆ 0648      ┆ 1983-02-14;1990-01-01 │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ mallesh   ┆ yamulla    ┆ 1234;4567 ┆ 1990-10-11            │
└───────────┴────────────┴───────────┴───────────────────────┘

Edit: eliminating empty strings and null values from rollup

We can add a filter step just before the arr.join to filter out both null and empty string "" values.

(
    my_dt.groupby(["last_name", "first_name"])
    .agg([pl.all().unique().sort().cast(pl.Utf8)])
    .with_columns(
        [
            pl.exclude(["last_name", "first_name"])
            .arr.eval(
                pl.element().filter(pl.element().is_not_null() & (pl.element() != ""))
            )
            .arr.join(";")
        ]
    )
)