`row_index()` was also recently added.

  df.with_columns(pl.col.b + pl.row_index().is_between(1, 3))
  # shape: (5, 2)
  # ┌─────┬─────┐
  # │ a   ┆ b   │
  # │ --- ┆ --- │
  # │ i64 ┆ i64 │
  # ╞═════╪═════╡
  # │ 1   ┆ 10  │
  # │ 2   ┆ 21  │
  # │ 3   ┆ 31  │
  # │ 4   ┆ 41  │
  # │ 5   ┆ 50  │
  # └─────┴─────┘

> Polars has an optimization to overwite a single value

I believe it is just "syntax sugar" for calling `Series.scatter()`[1]

> it doesn't allow slicing

I believe you are correct:

  df_polars[1:3, "b"] += 1
  # TypeError: cannot use "slice(1, 3, None)" for indexing

You can do:

  df_polars[list(range(1, 4)), "b"] += 1

Perhaps nobody has requested slice syntax? It seems like it would be easy to add.

[1]: https://github.com/pola-rs/polars/blob/9079e20ae59f8c75dcce8...