Struct csv_async::AsyncSerializer

pub struct AsyncSerializer<W: AsyncWrite + Unpin> { /* private fields */ }

An already configured CSV serde serializer.

A CSV serializer takes as input Rust structures that implement serde::Serialize trait and writes those data in a valid CSV 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 AsyncWriterBuilder. However, a AsyncSerializer has convenient constructor (from_writer) that use the default configuration.

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

Implementations

impl<W: AsyncWrite + Unpin> AsyncSerializer<W>

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

Build a CSV serializer with a default configuration that writes data to ser.

Note that the CSV serializer is buffered automatically, so you should not wrap ser in a buffered writer.

Example

use std::error::Error;
use csv_async::AsyncSerializer;
use serde::Serialize;

#[derive(Serialize)]
struct Row<'a> {
    name: &'a str,
    x: u64,
    y: u64,
}

async fn example() -> Result<(), Box<dyn Error>> {
    let mut ser = AsyncSerializer::from_writer(vec![]);
    ser.serialize(Row {name: "p1", x: 1, y: 2}).await?;
    ser.serialize(Row {name: "p2", x: 3, y: 4}).await?;

    let data = String::from_utf8(ser.into_inner().await?)?;
    assert_eq!(data, "name,x,y\np1,1,2\np2,3,4\n");
    Ok(())
}

pub async 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_async::AsyncSerializer;
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,
}

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

    let data = String::from_utf8(ser.into_inner().await?)?;
    assert_eq!(data, indoc::indoc! {"
        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_async::AsyncSerializer;
use serde::Serialize;

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

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

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

    let data = String::from_utf8(ser.into_inner().await?)?;
    assert_eq!(data, indoc::indoc! {"
        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_async::AsyncWriterBuilder;
use serde::Serialize;

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

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

    let data = String::from_utf8(ser.into_inner().await?)?;
    assert_eq!(data, indoc::indoc! {"
        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 async fn flush(&mut self) -> Result<()>

Flushes the underlying asynchronous writer.

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

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

Trait Implementations

impl<W: Debug + AsyncWrite + Unpin> Debug for AsyncSerializer<W>

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

Formats the value using the given formatter.

impl<W: AsyncWrite + Unpin> Drop for AsyncSerializer<W>

fn drop(&mut self)

Executes the destructor for this type.