Struct polars_io::csv::write::WriterBuilder

pub struct WriterBuilder { /* fields omitted */ }

This is supported on crate features csv-file and io_csv_write only.

Builds a CSV writer with various configuration knobs.

This builder can be used to tweak the field delimiter, record terminator and more. Once a CSV Writer is built, its configuration cannot be changed.

Implementations

impl WriterBuilder

pub fn new() -> WriterBuilder

Create a new builder for configuring CSV writing.

To convert a builder into a writer, call one of the methods starting with from_.

Example

use std::error::Error;
use csv::WriterBuilder;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new().from_writer(vec![]);
    wtr.write_record(&["a", "b", "c"])?;
    wtr.write_record(&["x", "y", "z"])?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "a,b,c\nx,y,z\n");
    Ok(())
}

pub fn from_path<P>(&self, path: P) -> Result<Writer<File>, Error> where
    P: AsRef<Path>, 

Build a CSV writer from this configuration that writes data to the given file path. The file is truncated if it already exists.

If there was a problem opening the file at the given path, then this returns the corresponding error.

Example

use std::error::Error;
use csv::WriterBuilder;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new().from_path("foo.csv")?;
    wtr.write_record(&["a", "b", "c"])?;
    wtr.write_record(&["x", "y", "z"])?;
    wtr.flush()?;
    Ok(())
}

pub fn from_writer<W>(&self, wtr: W) -> Writer<W> where
    W: Write, 

Build a CSV writer from this configuration that writes data to wtr.

Note that the CSV writer is buffered automatically, so you should not wrap wtr in a buffered writer like io::BufWriter.

Example

use std::error::Error;
use csv::WriterBuilder;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new().from_writer(vec![]);
    wtr.write_record(&["a", "b", "c"])?;
    wtr.write_record(&["x", "y", "z"])?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "a,b,c\nx,y,z\n");
    Ok(())
}

pub fn delimiter(&mut self, delimiter: u8) -> &mut WriterBuilder

The field delimiter to use when writing CSV.

The default is b','.

Example

use std::error::Error;
use csv::WriterBuilder;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new()
        .delimiter(b';')
        .from_writer(vec![]);
    wtr.write_record(&["a", "b", "c"])?;
    wtr.write_record(&["x", "y", "z"])?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "a;b;c\nx;y;z\n");
    Ok(())
}

pub fn has_headers(&mut self, yes: bool) -> &mut WriterBuilder

Whether to write a header row before writing any other row.

When this is enabled and the serialize method is used to write data with something that contains field names (i.e., a struct), then a header row is written containing the field names before any other row is written.

This option has no effect when using other methods to write rows. That is, if you don’t use serialize, then you must write your header row explicitly if you want a header row.

This is enabled by default.

Example: with headers

This shows how the header will be automatically written from the field names of a struct.

use std::error::Error;

use csv::WriterBuilder;
use serde::Serialize;

#[derive(Serialize)]
struct Row<'a> {
    city: &'a str,
    country: &'a str,
    // Serde allows us to name our headers exactly,
    // even if they don't match our struct field names.
    #[serde(rename = "popcount")]
    population: u64,
}

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new().from_writer(vec![]);
    wtr.serialize(Row {
        city: "Boston",
        country: "United States",
        population: 4628910,
    })?;
    wtr.serialize(Row {
        city: "Concord",
        country: "United States",
        population: 42695,
    })?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "\
city,country,popcount
Boston,United States,4628910
Concord,United States,42695
");
    Ok(())
}

Example: without headers

This shows that serializing things that aren’t structs (in this case, a tuple struct) won’t result in a header row being written. This means you usually don’t need to set has_headers(false) unless you explicitly want to both write custom headers and serialize structs.

use std::error::Error;
use csv::WriterBuilder;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new().from_writer(vec![]);
    wtr.serialize(("Boston", "United States", 4628910))?;
    wtr.serialize(("Concord", "United States", 42695))?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "\
Boston,United States,4628910
Concord,United States,42695
");
    Ok(())
}

pub fn flexible(&mut self, yes: bool) -> &mut WriterBuilder

Whether the number of fields in records is allowed to change or not.

When disabled (which is the default), writing CSV data will return an error if a record is written with a number of fields different from the number of fields written in a previous record.

When enabled, this error checking is turned off.

Example: writing flexible records

use std::error::Error;
use csv::WriterBuilder;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new()
        .flexible(true)
        .from_writer(vec![]);
    wtr.write_record(&["a", "b"])?;
    wtr.write_record(&["x", "y", "z"])?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "a,b\nx,y,z\n");
    Ok(())
}

Example: error when flexible is disabled

use std::error::Error;
use csv::WriterBuilder;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new()
        .flexible(false)
        .from_writer(vec![]);
    wtr.write_record(&["a", "b"])?;
    let err = wtr.write_record(&["x", "y", "z"]).unwrap_err();
    match *err.kind() {
        csv::ErrorKind::UnequalLengths { expected_len, len, .. } => {
            assert_eq!(expected_len, 2);
            assert_eq!(len, 3);
        }
        ref wrong => {
            panic!("expected UnequalLengths but got {:?}", wrong);
        }
    }
    Ok(())
}

pub fn terminator(&mut self, term: Terminator) -> &mut WriterBuilder

The record terminator to use when writing CSV.

A record terminator can be any single byte. The default is \n.

Note that RFC 4180 specifies that record terminators should be \r\n. To use \r\n, use the special Terminator::CRLF value.

Example: CRLF

This shows how to use RFC 4180 compliant record terminators.

use std::error::Error;
use csv::{Terminator, WriterBuilder};

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new()
        .terminator(Terminator::CRLF)
        .from_writer(vec![]);
    wtr.write_record(&["a", "b", "c"])?;
    wtr.write_record(&["x", "y", "z"])?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "a,b,c\r\nx,y,z\r\n");
    Ok(())
}

pub fn quote_style(&mut self, style: QuoteStyle) -> &mut WriterBuilder

The quoting style to use when writing CSV.

By default, this is set to QuoteStyle::Necessary, which will only use quotes when they are necessary to preserve the integrity of data.

Note that unless the quote style is set to Never, an empty field is quoted if it is the only field in a record.

Example: non-numeric quoting

This shows how to quote non-numeric fields only.

use std::error::Error;
use csv::{QuoteStyle, WriterBuilder};

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new()
        .quote_style(QuoteStyle::NonNumeric)
        .from_writer(vec![]);
    wtr.write_record(&["a", "5", "c"])?;
    wtr.write_record(&["3.14", "y", "z"])?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "\"a\",5,\"c\"\n3.14,\"y\",\"z\"\n");
    Ok(())
}

Example: never quote

This shows how the CSV writer can be made to never write quotes, even if it sacrifices the integrity of the data.

use std::error::Error;
use csv::{QuoteStyle, WriterBuilder};

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new()
        .quote_style(QuoteStyle::Never)
        .from_writer(vec![]);
    wtr.write_record(&["a", "foo\nbar", "c"])?;
    wtr.write_record(&["g\"h\"i", "y", "z"])?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "a,foo\nbar,c\ng\"h\"i,y,z\n");
    Ok(())
}

pub fn quote(&mut self, quote: u8) -> &mut WriterBuilder

The quote character to use when writing CSV.

The default is b'"'.

Example

use std::error::Error;
use csv::WriterBuilder;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new()
        .quote(b'\'')
        .from_writer(vec![]);
    wtr.write_record(&["a", "foo\nbar", "c"])?;
    wtr.write_record(&["g'h'i", "y\"y\"y", "z"])?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "a,'foo\nbar',c\n'g''h''i',y\"y\"y,z\n");
    Ok(())
}

pub fn double_quote(&mut self, yes: bool) -> &mut WriterBuilder

Enable double quote escapes.

This is enabled by default, but it may be disabled. When disabled, quotes in field data are escaped instead of doubled.

Example

use std::error::Error;
use csv::WriterBuilder;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new()
        .double_quote(false)
        .from_writer(vec![]);
    wtr.write_record(&["a", "foo\"bar", "c"])?;
    wtr.write_record(&["x", "y", "z"])?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "a,\"foo\\\"bar\",c\nx,y,z\n");
    Ok(())
}

pub fn escape(&mut self, escape: u8) -> &mut WriterBuilder

The escape character to use when writing CSV.

In some variants of CSV, quotes are escaped using a special escape character like \ (instead of escaping quotes by doubling them).

By default, writing these idiosyncratic escapes is disabled, and is only used when double_quote is disabled.

Example

use std::error::Error;
use csv::WriterBuilder;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new()
        .double_quote(false)
        .escape(b'$')
        .from_writer(vec![]);
    wtr.write_record(&["a", "foo\"bar", "c"])?;
    wtr.write_record(&["x", "y", "z"])?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "a,\"foo$\"bar\",c\nx,y,z\n");
    Ok(())
}

pub fn buffer_capacity(&mut self, capacity: usize) -> &mut WriterBuilder

Set the capacity (in bytes) of the internal buffer used in the CSV writer. This defaults to a reasonable setting.

Trait Implementations

impl Debug for WriterBuilder

pub fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl Default for WriterBuilder

pub fn default() -> WriterBuilder

Returns the “default value” for a type.