Struct Writer

pub struct Writer<W: Write> { /* private fields */ }

An already configured CSV writer.

A CSV writer takes as input Rust values and writes those values in a valid CSV format as output.

While CSV writing is considerably easier than parsing CSV, a proper writer will do a number of things for you:

1. Quote fields when necessary.
2. Check that all records have the same number of fields.
3. Write records with a single empty field correctly.
4. Automatically serialize normal Rust types to CSV records. When that type is a struct, a header row is automatically written corresponding to the fields of that struct.
5. Use buffering intelligently and otherwise avoid allocation. (This means that callers should not do their own buffering.)

All of the above can be configured using a WriterBuilder. However, a Writer has a couple of convenience constructors (from_path and from_writer) that use the default configuration.

Note that the default configuration of a Writer uses \n for record terminators instead of \r\n as specified by RFC 4180. Use the terminator method on WriterBuilder to set the terminator to \r\n if it’s desired.

Implementations

impl Writer<File>

pub fn from_path<P: AsRef<Path>>(path: P) -> Result<Writer<File>>

Build a CSV writer with a default 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::Writer;

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

impl<W: Write> Writer<W>

pub fn from_writer(wtr: W) -> Writer<W>

Build a CSV writer with a default 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::Writer;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = Writer::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 serialize<S: Serialize>(&mut self, record: S) -> Result<()>

Serialize a single record using Serde.

Example

This shows how to serialize normal Rust structs as CSV records. The fields of the struct are used to write a header row automatically. (Writing the header row automatically can be disabled by building the CSV writer with a WriterBuilder and calling the has_headers method.)

use std::error::Error;

use csv::Writer;

#[derive(serde::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 = Writer::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(())
}

Rules

The behavior of serialize is fairly simple:

1. Nested containers (tuples, Vecs, structs, etc.) are always flattened (depth-first order).

2. If has_headers is true and the type contains field names, then a header row is automatically generated.

However, some container types cannot be serialized, and if has_headers is true, there are some additional restrictions on the types that can be serialized. See below for details.

For the purpose of this section, Rust types can be divided into three categories: scalars, non-struct containers, and structs.

Scalars

Single values with no field names are written like the following. Note that some of the outputs may be quoted, according to the selected quoting style.

Name	Example Type	Example Value	Output
boolean	bool	true	true
integers	i8, i16, i32, i64, i128, u8, u16, u32, u64, u128	5	5
floats	f32, f64	3.14	3.14
character	char	'☃'	☃
string	&str	"hi"	hi
bytes	&[u8]	b"hi"[..]	hi
option	Option	None	empty
option		Some(5)	5
unit	()	()	empty
unit struct	struct Foo;	Foo	Foo
unit enum variant	enum E { A, B }	E::A	A
newtype struct	struct Foo(u8);	Foo(5)	5
newtype enum variant	enum E { A(u8) }	E::A(5)	5

Note that this table includes simple structs and enums. For example, to serialize a field from either an integer or a float type, one can do this:

use std::error::Error;

use csv::Writer;

#[derive(serde::Serialize)]
struct Row {
    label: String,
    value: Value,
}

#[derive(serde::Serialize)]
enum Value {
    Integer(i64),
    Float(f64),
}

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = Writer::from_writer(vec![]);
    wtr.serialize(Row {
        label: "foo".to_string(),
        value: Value::Integer(3),
    })?;
    wtr.serialize(Row {
        label: "bar".to_string(),
        value: Value::Float(3.14),
    })?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "\
label,value
foo,3
bar,3.14
");
    Ok(())
}

Non-Struct Containers

Nested containers are flattened to their scalar components, with the exception of a few types that are not allowed:

Name	Example Type	Example Value	Output
sequence	Vec<u8>	vec![1, 2, 3]	1,2,3
tuple	(u8, bool)	(5, true)	5,true
tuple struct	Foo(u8, bool)	Foo(5, true)	5,true
tuple enum variant	enum E { A(u8, bool) }	E::A(5, true)	error
struct enum variant	enum E { V { a: u8, b: bool } }	E::V { a: 5, b: true }	error
map	BTreeMap<K, V>	BTreeMap::new()	error

Structs

Like the other containers, structs are flattened to their scalar components:

Name	Example Type	Example Value	Output
struct	struct Foo { a: u8, b: bool }	Foo { a: 5, b: true }	5,true

If has_headers is false, then there are no additional restrictions; types can be nested arbitrarily. For example:

use std::error::Error;

use csv::WriterBuilder;

#[derive(serde::Serialize)]
struct Row {
    label: String,
    values: Vec<f64>,
}

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = WriterBuilder::new()
        .has_headers(false)
        .from_writer(vec![]);
    wtr.serialize(Row {
        label: "foo".to_string(),
        values: vec![1.1234, 2.5678, 3.14],
    })?;

    let data = String::from_utf8(wtr.into_inner()?)?;
    assert_eq!(data, "\
foo,1.1234,2.5678,3.14
");
    Ok(())
}

However, if has_headers were enabled in the above example, then serialization would return an error. Specifically, when has_headers is true, there are two restrictions:

1. Named field values in structs must be scalars.

2. All scalars must be named field values in structs.

Other than these two restrictions, types can be nested arbitrarily. Here are a few examples:

Value	Header	Record
(Foo { x: 5, y: 6 }, Bar { z: true })	x,y,z	5,6,true
vec![Foo { x: 5, y: 6 }, Foo { x: 7, y: 8 }]	x,y,x,y	5,6,7,8
(Foo { x: 5, y: 6 }, vec![Bar { z: Baz(true) }])	x,y,z	5,6,true
Foo { x: 5, y: (6, 7) }	error: restriction 1	5,6,7
(5, Foo { x: 6, y: 7 }	error: restriction 2	5,6,7
(Foo { x: 5, y: 6 }, true)	error: restriction 2	5,6,true

pub fn write_record<I, T>(&mut self, record: I) -> Result<()>

where I: IntoIterator<Item = T>, T: AsRef<[u8]>,

Write a single record.

This method accepts something that can be turned into an iterator that yields elements that can be represented by a &[u8].

This may be called with an empty iterator, which will cause a record terminator to be written. If no fields had been written, then a single empty field is written before the terminator.

Example

use std::error::Error;
use csv::Writer;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = Writer::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 write_byte_record(&mut self, record: &ByteRecord) -> Result<()>

Write a single ByteRecord.

This method accepts a borrowed ByteRecord and writes its contents to the underlying writer.

This is similar to write_record except that it specifically requires a ByteRecord. This permits the writer to possibly write the record more quickly than the more generic write_record.

This may be called with an empty record, which will cause a record terminator to be written. If no fields had been written, then a single empty field is written before the terminator.

Example

use std::error::Error;
use csv::{ByteRecord, Writer};

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

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

pub fn write_field<T: AsRef<[u8]>>(&mut self, field: T) -> Result<()>

Write a single field.

One should prefer using write_record over this method. It is provided for cases where writing a field at a time is more convenient than writing a record at a time.

Note that if this API is used, write_record should be called with an empty iterator to write a record terminator.

Example

use std::error::Error;
use csv::Writer;

fn example() -> Result<(), Box<dyn Error>> {
    let mut wtr = Writer::from_writer(vec![]);
    wtr.write_field("a")?;
    wtr.write_field("b")?;
    wtr.write_field("c")?;
    wtr.write_record(None::<&[u8]>)?;
    wtr.write_field("x")?;
    wtr.write_field("y")?;
    wtr.write_field("z")?;
    wtr.write_record(None::<&[u8]>)?;

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

pub fn flush(&mut self) -> Result<()>

Flush the contents of the internal buffer to the underlying writer.

If there was a problem writing to the underlying writer, then an error is returned.

Note that this also flushes the underlying writer.

pub fn into_inner(self) -> Result<W, IntoInnerError<Writer<W>>>

Flush the contents of the internal buffer and return the underlying writer.

Trait Implementations

impl<W: Debug + Write> Debug for Writer<W>

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

Formats the value using the given formatter.

impl<W: Write> Drop for Writer<W>

fn drop(&mut self)

Executes the destructor for this type.