How to swap column values on conditions in python polars?

I have a data frame as below:

df_n = pl.from_pandas(pd.DataFrame({'last_name':[np.nan,'mallesh','bhavik'],
                                    'first_name':['a','b','c'],
                                    'middle_name_or_initial':['aa','bb','cc']}))

Here I would like to find an observation which has First and Middle Name not NULL and Last Name is Null, in this case first_name should be swapped to last_name and middle_name should be swapped to first_name, and middle_name to be EMPTY.

expected output will be:

I’m trying with this command:

df_n.with_columns([
    pl.when((pl.col('first_name').is_not_null()) & (pl.col('middle_name_or_initial').is_not_null()) & (pl.col('last_name').is_null())
           ).then(pl.col('first_name').alias('last_name')).otherwise(pl.col('last_name').alias('first_name')),
    pl.when((pl.col('first_name').is_not_null()) & (pl.col('middle_name_or_initial').is_not_null()) & (pl.col('last_name').is_null())
           ).then(pl.col('middle_name_or_initial').alias('first_name')).otherwise('').alias('middle_name_or_initial')
]
)

Here it is throwing a wrong output and any help ?

>Solution :

With pl.when().then().otherwise() you create values for only one column (so only one alias at the end is allowed).

In [67]: df_n.with_columns(
    ...:     [
    ...:         # Create temp column with filter, so it does not have to be recalculated 3 times.
    ...:         ((pl.col('first_name').is_not_null()) & (pl.col('middle_name_or_initial').is_not_null()) & (pl.col('last_name').is_null())).alias("swap_names")
    ...:     ]
    ...: ).with_columns(
    ...:     [
    ...:         pl.when(pl.col("swap_names")).then(pl.col("first_name")).otherwise(pl.col("last_name")).alias("last_name_new"),
    ...:         pl.when(pl.col("swap_names")).then(pl.col("middle_name_or_initial")).otherwise(pl.col("first_name")).alias("first_name_new"),
    ...:         pl.when(pl.col("swap_names")).then(None).otherwise(pl.col("middle_name_or_initial")).alias("middle_name_or_initial_new"),
    ...:     ]
    ...: )
Out[67]: 
shape: (3, 7)
┌───────────┬────────────┬────────────────────────┬────────────┬───────────────┬────────────────┬────────────────────────────┐
│ last_name ┆ first_name ┆ middle_name_or_initial ┆ swap_names ┆ last_name_new ┆ first_name_new ┆ middle_name_or_initial_new │
│ ---       ┆ ---        ┆ ---                    ┆ ---        ┆ ---           ┆ ---            ┆ ---                        │
│ str       ┆ str        ┆ str                    ┆ bool       ┆ str           ┆ str            ┆ str                        │
╞═══════════╪════════════╪════════════════════════╪════════════╪═══════════════╪════════════════╪════════════════════════════╡
│ null      ┆ a          ┆ aa                     ┆ true       ┆ a             ┆ aa             ┆ null                       │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ mallesh   ┆ b          ┆ bb                     ┆ false      ┆ mallesh       ┆ b              ┆ bb                         │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ bhavik    ┆ c          ┆ cc                     ┆ false      ┆ bhavik        ┆ c              ┆ cc                         │
└───────────┴────────────┴────────────────────────┴────────────┴───────────────┴────────────────┴────────────────────────────┘