polars_ethan 0.2.2

Fast Polars functions that Ethan likes to use.

Features

Demean

The demean function subtracts the mean from each non-NaN value in an array, preserving NaN values.

import polars as pl
from ethanpolars import demean

# Sample data...
cameras = pl.DataFrame(
    {
        "brand": ["Leica", "Leica", "Leica", "Mamiya", "Mamiya", "Mamiya"],
        "model": ["MP", "M4", "M7", "RB67", "7II", "645 Pro"],
        "price": [6000, 10000, None, 600, 3000, 900],
    }
)

cameras = cameras.with_columns(
    pl.col('price')
    .fill_null(np.nan)
    .map_batches(demean)
    .over(pl.col("brand"))
    .fill_nan(None)
    .alias('d_price')
)

print(cameras)

┌────────┬─────────┬───────┬─────────┐ │ brand ┆ model ┆ price ┆ d_price │ │ --- ┆ --- ┆ --- ┆ --- │ │ str ┆ str ┆ i64 ┆ f64 │ ╞════════╪═════════╪═══════╪═════════╡ │ Leica ┆ MP ┆ 6000 ┆ -2000.0 │ │ Leica ┆ M4 ┆ 10000 ┆ 2000.0 │ │ Leica ┆ M7 ┆ null ┆ null │ │ Mamiya ┆ RB67 ┆ 600 ┆ -900.0 │ │ Mamiya ┆ 7II ┆ 3000 ┆ 1500.0 │ │ Mamiya ┆ 645 Pro ┆ 900 ┆ -600.0 │ └────────┴─────────┴───────┴─────────┘

Note how null values must be converted to NaN before being passed to the function.
Generalized ufuncs do not except null values, but NaN's are fine and behave as we would like.