# Mutate the Table in a Validation Step

[TABLE]

``` python
import pointblank as pb
import polars as pl
import narwhals as nw

# Define preprocessing functions
def get_median_a(df):
    """Use a Polars expression to aggregate column `a`."""
    return df.select(pl.median("a"))

def add_b_length_column(df):
    """Use Narwhals to add a string length column `b_len`."""
    return (
        nw.from_native(df)
        .with_columns(b_len=nw.col("b").str.len_chars())
    )

validation = (
    pb.Validate(
        data=pb.load_dataset(dataset="small_table", tbl_type="polars")
    )
    .col_vals_between(
        columns="a",
        left=3, right=6,
        pre=get_median_a
    )
    .col_vals_eq(
        columns="b_len",
        value=9,
        pre=add_b_length_column
    )
    .interrogate()
)

validation
```

Preview of Input Table

| PolarsRows13Columns8 |  |  |  |  |  |  |  |  |
|----|----|----|----|----|----|----|----|----|
|  | date_time *Datetime* | date *Date* | a *Int64* | b *String* | c *Int64* | d *Float64* | e *Boolean* | f *String* |
| 1 | 2016-01-04 11:00:00 | 2016-01-04 | 2 | 1-bcd-345 | 3 | 3423.29 | True | high |
| 2 | 2016-01-04 00:32:00 | 2016-01-04 | 3 | 5-egh-163 | 8 | 9999.99 | True | low |
| 3 | 2016-01-05 13:32:00 | 2016-01-05 | 6 | 8-kdg-938 | 3 | 2343.23 | True | high |
| 4 | 2016-01-06 17:23:00 | 2016-01-06 | 2 | 5-jdo-903 | None | 3892.4 | False | mid |
| 5 | 2016-01-09 12:36:00 | 2016-01-09 | 8 | 3-ldm-038 | 7 | 283.94 | True | low |
| 6 | 2016-01-11 06:15:00 | 2016-01-11 | 4 | 2-dhe-923 | 4 | 3291.03 | True | mid |
| 7 | 2016-01-15 18:46:00 | 2016-01-15 | 7 | 1-knw-093 | 3 | 843.34 | True | high |
| 8 | 2016-01-17 11:27:00 | 2016-01-17 | 4 | 5-boe-639 | 2 | 1035.64 | False | low |
| 9 | 2016-01-20 04:30:00 | 2016-01-20 | 3 | 5-bce-642 | 9 | 837.93 | False | high |
| 10 | 2016-01-20 04:30:00 | 2016-01-20 | 3 | 5-bce-642 | 9 | 837.93 | False | high |
| 11 | 2016-01-26 20:07:00 | 2016-01-26 | 4 | 2-dmx-010 | 7 | 833.98 | True | low |
| 12 | 2016-01-28 02:51:00 | 2016-01-28 | 2 | 7-dmx-010 | 8 | 108.34 | False | low |
| 13 | 2016-01-30 11:23:00 | 2016-01-30 | 1 | 3-dka-303 | None | 2230.09 | True | high |