Access newly created column in .with_columns() when using polars

Question

I am new to Polars and I am not sure whether I am using .with_columns() correctly.

Here's a situation I encounter frequently: There's a dataframe and in .with_columns(), I apply some operation to a column. For example, I convert some dates from str to date type and then want to compute the duration between start and end date. I'd implement this as follows.

import polars as pl 

pl.DataFrame(
    {
        "start": ["01.01.2019", "01.01.2020"],
        "end": ["11.01.2019", "01.05.2020"],
    }
).with_columns(
    pl.col("start").str.to_date(),
    pl.col("end").str.to_date(),
).with_columns(
    (pl.col("end") - pl.col("start")).alias("duration"),
)

First, I convert the two columns, next I call .with_columns() again.

Something shorter like this does not work:

pl.DataFrame(
    {
        "start": ["01.01.2019", "01.01.2020"],
        "end": ["11.01.2019", "01.05.2020"],
    }
).with_columns(
    pl.col("start").str.to_date(),
    pl.col("end").str.to_date(),
    (pl.col("end") - pl.col("start")).alias("duration"),
)

# InvalidOperationError: sub operation not supported for dtypes `str` and `str`

Is there a way to avoid calling .with_columns() twice and to write this in a more compact way?

Answer 1

The second .with_columns() is needed.

From the GitHub Issues

I don't want this extra complexity in polars. If you want to use an updated column, you need two with_columns. This makes it much more readable, simple, and explainable.

In the given example, passing multiple names to col() could simplify it slightly.

(df.with_columns(pl.col("start", "end").str.to_date())
   .with_columns(duration = pl.col("end") - pl.col("start"))
)

shape: (2, 3)
┌────────────┬────────────┬──────────────┐
│ start      ┆ end        ┆ duration     │
│ ---        ┆ ---        ┆ ---          │
│ date       ┆ date       ┆ duration[ms] │
╞════════════╪════════════╪══════════════╡
│ 2019-01-01 ┆ 2019-01-11 ┆ 10d          │
│ 2020-01-01 ┆ 2020-05-01 ┆ 121d         │
└────────────┴────────────┴──────────────┘