8. I'd like to have an argument, please

Say you want to rewrite

def add_suffix(s, *, suffix):
    return s + suffix

s.map_elements(lambda x: add_suffix(x, suffix='-billy'))

as a plugin. How can you do that?

We've covered passing in extra columns, but...how about passing extra keyword arguments?

We'll do this with kwargs. In minimal_plugin/__init__.py, add the following:

def add_suffix(expr: IntoExprColumn, *, suffix: str) -> pl.Expr:
    return register_plugin_function(
        args=[expr],
        plugin_path=LIB,
        function_name="add_suffix",
        is_elementwise=True,
        kwargs={"suffix": suffix},
    )

In src/expressions.rs, we'll then first have to define a struct to hold our keyword-arguments:

#[derive(Deserialize)]
struct AddSuffixKwargs {
    suffix: String,
}

Make sure to also add

use serde::Deserialize;

to the top of the file.

Then, we can just pass an argument of this type to a add_suffix function, which is going to be very similar to the good version of pig_latinnify:

#[polars_expr(output_type=String)]
fn add_suffix(inputs: &[Series], kwargs: AddSuffixKwargs) -> PolarsResult<Series> {
    let s = &inputs[0];
    let ca = s.str()?;
    let out = ca.apply_into_string_amortized(|value, output| {
        write!(output, "{}{}", value, kwargs.suffix).unwrap();
    });
    Ok(out.into_series())
}

To see it in action, compile with maturin develop (or maturin develop --release if you're benchmarking), and then you should be able to put

import polars as pl
import minimal_plugin as mp

df = pl.DataFrame({'a': ['bob', 'billy']})
print(df.with_columns(mp.add_suffix('a', suffix='-billy')))

into run.py, and run it to get

shape: (2, 1)
┌─────────────┐
│ a           │
│ ---         │
│ str         │
╞═════════════╡
│ bob-billy   │
│ billy-billy │
└─────────────┘

You can add multiple keyword-arguments in the same function, just make sure to include them in the struct which you define on the Rust side.