Polars selectors alias with when/then/otherwise

Question

Say I have this:

import polars as pl
import polars.selectors as cs

df = pl.select(
    j = pl.int_range(10, 99).sample(10, with_replacement=True),
    k = pl.int_range(10, 99).sample(10, with_replacement=True),
    l = pl.int_range(10, 99).sample(10, with_replacement=True),
)

shape: (10, 3)
┌─────┬─────┬─────┐
│ j   ┆ k   ┆ l   │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ i64 │
╞═════╪═════╪═════╡
│ 71  ┆ 79  ┆ 67  │
│ 26  ┆ 42  ┆ 55  │
│ 12  ┆ 43  ┆ 85  │
│ 92  ┆ 96  ┆ 14  │
│ 95  ┆ 26  ┆ 62  │
│ 75  ┆ 14  ┆ 56  │
│ 61  ┆ 41  ┆ 75  │
│ 74  ┆ 97  ┆ 70  │
│ 73  ┆ 32  ┆ 10  │
│ 66  ┆ 98  ┆ 40  │
└─────┴─────┴─────┘

and I want to apply the same when/then/otherwise condition on multiple columns:

df.select(
    pl.when(cs.numeric() < 50)
      .then(1)
      .otherwise(2)
)

This fails with:

DuplicateError: the name 'literal' is duplicate

How do I make this use the currently selected column as the alias? I.e. I want the equivalent of this:

df.select(
    pl.when(pl.col(c) < 50)
      .then(1)
      .otherwise(2)
      .alias(c)
    for c in df.columns
)

shape: (10, 3)
┌─────┬─────┬─────┐
│ j   ┆ k   ┆ l   │
│ --- ┆ --- ┆ --- │
│ i32 ┆ i32 ┆ i32 │
╞═════╪═════╪═════╡
│ 2   ┆ 2   ┆ 2   │
│ 1   ┆ 1   ┆ 2   │
│ 1   ┆ 1   ┆ 2   │
│ 2   ┆ 2   ┆ 1   │
│ 2   ┆ 1   ┆ 2   │
│ 2   ┆ 1   ┆ 2   │
│ 2   ┆ 1   ┆ 2   │
│ 2   ┆ 2   ┆ 2   │
│ 2   ┆ 1   ┆ 1   │
│ 2   ┆ 2   ┆ 1   │
└─────┴─────┴─────┘

Answer 1

You can use .name.keep()

df.select(
   pl.when(cs.numeric() < 50)
     .then(1)
     .otherwise(2)
     .name.keep()
)

shape: (10, 3)
┌─────┬─────┬─────┐
│ j   ┆ k   ┆ l   │
│ --- ┆ --- ┆ --- │
│ i32 ┆ i32 ┆ i32 │
╞═════╪═════╪═════╡
│ 1   ┆ 1   ┆ 2   │
│ 2   ┆ 1   ┆ 1   │
│ 1   ┆ 2   ┆ 1   │
│ 2   ┆ 2   ┆ 1   │
│ 2   ┆ 1   ┆ 2   │
│ 2   ┆ 2   ┆ 2   │
│ 2   ┆ 2   ┆ 1   │
│ 2   ┆ 2   ┆ 2   │
│ 1   ┆ 1   ┆ 2   │
│ 2   ┆ 2   ┆ 1   │
└─────┴─────┴─────┘