How can I show all columns of a dataframe with many columns?

I particularly like using rounded corners.
For example, I use this config env:

use polars::prelude::*;
use std::{
    env,
    error::Error,
};

fn main() -> Result<(), Box<dyn Error>> {
    // set MAX_COLS to 20
    configure_the_environment();

    // df with 12 columns
    let dataframe = df! [
        "Column A 12345678910" => ["a", "b", "c"],
        "Column B 12345678910" => [1, 2, 3],
        "Column C 12345678910" => [Some(1), None, Some(3)],
        "Column D 12345678910" => [Some(1), None, Some(3)],
        "Column E 12345678910" => [Some(1), None, Some(3)],
        "Column F 12345678910" => [1, 2, 3],
        "Column G 12345678910" => [1, 2, 3],
        "Column Z 12345678910" => [1, 2, 3],
        "Column Ex 12345678910" => [Some(1), None, Some(3)],
        "Column Fs 12345678910" => [1, 2, 3],
        "Column Ga 12345678910" => [1, 2, 3],
        "Column Zz 12345678910" => [1, 2, 3],
    ]?;

    println!("{:?}", dataframe);

    Ok(())
}

/// Configure Polars with ENV vars
pub fn configure_the_environment() {
    env::set_var("POLARS_FMT_TABLE_ROUNDED_CORNERS", "1"); // apply rounded corners to UTF8-styled tables.
    env::set_var("POLARS_FMT_MAX_COLS", "20"); // maximum number of columns shown when formatting DataFrames.
    env::set_var("POLARS_FMT_MAX_ROWS", "10"); // maximum number of rows shown when formatting DataFrames.
    env::set_var("POLARS_FMT_STR_LEN", "50");  // maximum number of characters printed per string value.
}

I wrote a helper function for my project that addresses this, that way I don't make global modifications if you only want to print the entire DataFrame in certain places in the code:

use polars::prelude::DataFrame;
use std::env;

pub fn print_full_df(df: &DataFrame, msg: Option<&str>) {
    // Save old settings
    let old_max_rows = env::var("POLARS_FMT_MAX_ROWS").ok();
    let old_max_cols = env::var("POLARS_FMT_MAX_COLS").ok();

    // Show everything
    env::set_var("POLARS_FMT_MAX_ROWS", "-1");
    env::set_var("POLARS_FMT_MAX_COLS", "-1");

    // Optional message, then DataFrame
    if let Some(m) = msg {
        println!("{}", m);
    }
    println!("{}", df);

    // Restore
    if let Some(val) = old_max_rows {
        env::set_var("POLARS_FMT_MAX_ROWS", val);
    } else {
        env::remove_var("POLARS_FMT_MAX_ROWS");
    }
    if let Some(val) = old_max_cols {
        env::set_var("POLARS_FMT_MAX_COLS", val);
    } else {
        env::remove_var("POLARS_FMT_MAX_COLS");
    }
}

You can use it like so:

// without message
print_full_df(&df, None);

// with a heading
print_full_df(&df, Some("Full DataFrame contents:"));

Optionally, you can have it truncate to the first x entries in the DF to avoid blowing up your logs with huge entries, if you are more interested in a subset of the data than the whole thing:

use polars::prelude::DataFrame;
use std::env;

pub fn print_full_df(df: &DataFrame, msg: Option<&str>) {
    // Save old POLARS_FMT settings
    let old_max_rows = env::var("POLARS_FMT_MAX_ROWS").ok();
    let old_max_cols = env::var("POLARS_FMT_MAX_COLS").ok();

    // Ensure we can see all columns (rows unlimited but we'll only print head)
    env::set_var("POLARS_FMT_MAX_ROWS", "-1");
    env::set_var("POLARS_FMT_MAX_COLS", "-1");

    // Optional message
    if let Some(m) = msg {
        println!("{}", m);
    }

    // Print only the first 5 rows (head will truncate to fewer if DF.len() < 5)
    let head_df = df.head(Some(5));
    println!("{}", head_df);

    // Restore previous settings
    if let Some(val) = old_max_rows {
        env::set_var("POLARS_FMT_MAX_ROWS", val);
    } else {
        env::remove_var("POLARS_FMT_MAX_ROWS");
    }
    if let Some(val) = old_max_cols {
        env::set_var("POLARS_FMT_MAX_COLS", val);
    } else {
        env::remove_var("POLARS_FMT_MAX_COLS");
    }
}