JSON files

Polars can read and write both standard JSON and newline-delimited JSON (NDJSON).

Read

JSON

Reading a JSON file should look familiar:

Python Rust

read_json

df = pl.read_json("docs/assets/data/path.json")

JsonReader · Available on feature json

use polars::prelude::*;

let mut file = std::fs::File::open("docs/assets/data/path.json").unwrap();
let df = JsonReader::new(&mut file).finish()?;

Newline Delimited JSON

JSON objects that are delimited by newlines can be read into Polars in a much more performant way than standard json.

Polars can read an NDJSON file into a DataFrame using the read_ndjson function:

Python Rust

read_ndjson

df = pl.read_ndjson("docs/assets/data/path.json")

JsonLineReader · Available on feature json

let mut file = std::fs::File::open("docs/assets/data/path.json").unwrap();
let df = JsonLineReader::new(&mut file).finish().unwrap();

Write

Python Rust

write_json · write_ndjson

df = pl.DataFrame({"foo": [1, 2, 3], "bar": [None, "bak", "baz"]})
df.write_json("docs/assets/data/path.json")

JsonWriter · JsonWriter · Available on feature json

let mut df = df!(
    "foo" => &[1, 2, 3],
    "bar" => &[None, Some("bak"), Some("baz")],
)
.unwrap();

let mut file = std::fs::File::create("docs/assets/data/path.json").unwrap();

// json
JsonWriter::new(&mut file)
    .with_json_format(JsonFormat::Json)
    .finish(&mut df)
    .unwrap();

// ndjson
JsonWriter::new(&mut file)
    .with_json_format(JsonFormat::JsonLines)
    .finish(&mut df)
    .unwrap();

Scan

Polars allows you to scan a JSON input only for newline delimited json. Scanning delays the actual parsing of the file and instead returns a lazy computation holder called a LazyFrame.

Python Rust

scan_ndjson

df = pl.scan_ndjson("docs/assets/data/path.json")

LazyJsonLineReader · Available on feature json

let lf = LazyJsonLineReader::new("docs/assets/data/path.json")
    .finish()
    .unwrap();