Read CSV file into Polars dataframe with Rust

Question

I would like to read the CSV file into a Polars dataframe.

Copying the code from the official documentation fails to run with cargo.

use polars::prelude::*;
use std::fs::File;

fn example() -> Result<DataFrame> {
    let file = File::open("iris.csv").expect("could not open file");

    CsvReader::new(file)
            .infer_schema(None)
            .has_header(true)
            .finish()
}

fn main() {
    let df = example();
    println!("{:?}", df);
}

results in

$ cargo run
   Compiling project v0.1.0
error[E0107]: this enum takes 2 generic arguments but 1 generic argument was supplied
 --> src/main.rs:4:17
  |
4 | fn example() -> Result<DataFrame> {
  |                 ^^^^^^ --------- supplied 1 generic argument
  |                 |
  |                 expected 2 generic arguments
  |
help: add missing generic argument
  |
4 | fn example() -> Result<DataFrame, E> {
  |                                 +++

For more information about this error, try `rustc --explain E0107`.
error: could not compile `eon-playground` due to previous error

Chayim Friedman · Accepted Answer · 2023-01-23 13:19:45Z

3

You were reading the docs of an old version (you can see that by the "Go to latest version" at the top). In this version, polars::prelude exported an alias of Result. Now it is renamed to PolarsResult, and the updated example is:

use polars_core::prelude::*;
use polars_io::prelude::*;
use std::fs::File;

fn example() -> PolarsResult<DataFrame> {
    CsvReader::from_path("iris_csv")?
            .has_header(true)
            .finish()
}

answered Jan 23, 2023 at 13:19

Chayim Friedman

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1 Comment

David Over a year ago

Thank you for this! It worked, however, in addition, the following import was unused: use std::fs::File;.

Larzsolice · Accepted Answer · 2023-12-23 09:57:54Z

I had further problems parsing the result (probably beginner problems, but sharing the solution in anyway). I was having compiler issues until I pre-initialised df as an empty DataFrame and then reassigned it in the match statement. This is probably not the only solution, but it gets you from CSV file to a DataFrame in Rust.


use polars::prelude::*;
// You should probably use polars_core rather

const TESTFILE: &str = "any.csv";
// Doesn't need to be a constant for you

fn read_csv(filename: &str) -> Result<DataFrame, PolarsError> {
    CsvReader::from_path(filename)?
        .has_header(true)
        .finish()
}

fn main() -> Result<(), PolarsError> {
    // * Import CSV
    let mut df = DataFrame::empty();
    match read_csv(TESTFILE) {
        Ok(x) => { df = x },
        Err(e) => { return Err(e) },
    };
     
    println!("{:?}", df.head(None));

    Ok(())
}

user459872 · Accepted Answer · 2024-11-10 14:47:50Z

2

With the latest version of rust polars(0.44.2), the accepted answer does not work for me. It seems like the API's have now changed.

use polars::prelude::*;

fn main() {
    let df = CsvReadOptions::default()
        .with_has_header(true)
        .try_into_reader_with_file_path(Some("iris.csv".into()))
        .unwrap()
        .finish()
        .unwrap();
    println!("{:?}", df);
}

You also must have the polars-io feature enabled in Cargo.toml.

[dependencies]
polars = { version = "0.44.2", features = ["polars-io"] }

edited Nov 10, 2024 at 14:47

answered Nov 7, 2024 at 2:22

user459872

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