Polars: how to add a column in front?

Question

What would be the most idiomatic (and efficient) way to add a column in front of a polars data frame? Same thing like .with_columns but add it at index 0?

Answer 1

Since version 1.9.0 you can do it with pl.DataFrame.insert_column() which supports expression now, which can refer to existing columns as well:

df = pl.DataFrame({
    "a":[1,2,3],
    "b": list("abc")
})


df.insert_column(
    0,
    (pl.col.a + 1).alias("c")
)

shape: (3, 3)
┌─────┬─────┬─────┐
│ c   ┆ a   ┆ b   │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ str │
╞═════╪═════╪═════╡
│ 2   ┆ 1   ┆ a   │
│ 3   ┆ 2   ┆ b   │
│ 4   ┆ 3   ┆ c   │
└─────┴─────┴─────┘