csvutil 
Package csvutil provides fast and idiomatic mapping between CSV and Go (golang) values.
This package does not provide a CSV parser itself, it is based on the Reader and Writer interfaces which are implemented by eg. std Go (golang) csv package. This gives a possibility of choosing any other CSV writer or reader which may be more performant.
Installation
go get github.com/jszwec/csvutil
Requirements
- Go1.7+
Index
Example
Unmarshal
Nice and easy Unmarshal is using the Go std csv.Reader with its default options. Use Decoder for streaming and more advanced use cases.
var csvInput = []byte(`
name,age,CreatedAt
jacek,26,2012-04-01T15:00:00Z
john,,0001-01-01T00:00:00Z`,
)
type User struct {
Name string `csv:"name"`
Age int `csv:"age,omitempty"`
CreatedAt time.Time
}
var users []User
if err := csvutil.Unmarshal(csvInput, &users); err != nil {
fmt.Println("error:", err)
}
for _, u := range users {
fmt.Printf("%+v\n", u)
}
// Output:
// {Name:jacek Age:26 CreatedAt:2012-04-01 15:00:00 +0000 UTC}
// {Name:john Age:0 CreatedAt:0001-01-01 00:00:00 +0000 UTC}
Marshal
Marshal is using the Go std csv.Writer with its default options. Use Encoder for streaming or to use a different Writer.
type Address struct {
City string
Country string
}
type User struct {
Name string
Address
Age int `csv:"age,omitempty"`
CreatedAt time.Time
}
users := []User{
{
Name: "John",
Address: Address{"Boston", "USA"},
Age: 26,
CreatedAt: time.Date(2010, 6, 2, 12, 0, 0, 0, time.UTC),
},
{
Name: "Alice",
Address: Address{"SF", "USA"},
},
}
b, err := csvutil.Marshal(users)
if err != nil {
fmt.Println("error:", err)
}
fmt.Println(string(b))
// Output:
// Name,City,Country,age,CreatedAt
// John,Boston,USA,26,2010-06-02T12:00:00Z
// Alice,SF,USA,,0001-01-01T00:00:00Z
Unmarshal and metadata
It may happen that your CSV input will not always have the same header. In addition to your base fields you may get extra metadata that you would still like to store. Decoder provides Unused method, which after each call to Decode can report which header indexes were not used during decoding. Based on that, it is possible to handle and store all these extra values.
type User struct {
Name string `csv:"name"`
City string `csv:"city"`
Age int `csv:"age"`
OtherData map[string]string `csv:"-"`
}
csvReader := csv.NewReader(strings.NewReader(`
name,age,city,zip
alice,25,la,90005
bob,30,ny,10005`))
dec, err := csvutil.NewDecoder(csvReader)
if err != nil {
log.Fatal(err)
}
header := dec.Header()
var users []User
for {
u := User{OtherData: make(map[string]string)}
if err := dec.Decode(&u); err == io.EOF {
break
} else if err != nil {
log.Fatal(err)
}
for _, i := range dec.Unused() {
u.OtherData[header[i]] = dec.Record()[i]
}
users = append(users, u)
}
fmt.Println(users)
// Output:
// [{alice la 25 map[zip:90005]} {bob ny 30 map[zip:10005]}]
But my CSV file has no header...
Some CSV files have no header, but if you know how it should look like, it is possible to define a struct and generate it. All that is left to do, is to pass it to a decoder.
type User struct {
ID int
Name string
Age int `csv:",omitempty"`
City string
}
csvReader := csv.NewReader(strings.NewReader(`
1,John,27,la
2,Bob,,ny`))
// in real application this should be done once in init function.
userHeader, err := csvutil.Header(User{}, "csv")
if err != nil {
log.Fatal(err)
}
dec, err := csvutil.NewDecoder(csvReader, userHeader...)
if err != nil {
log.Fatal(err)
}
var users []User
for {
var u User
if err := dec.Decode(&u); err == io.EOF {
break
} else if err != nil {
log.Fatal(err)
}
users = append(users, u)
}
fmt.Printf("%+v", users)
// Output:
// [{ID:1 Name:John Age:27 City:la} {ID:2 Name:Bob Age:0 City:ny}]
Decoder.Map - data normalization
The Decoder's Map function is a powerful tool that can help clean up or normalize the incoming data before the actual decoding takes place.
Lets say we want to decode some floats and the csv input contains some NaN values, but these values are represented by the 'n/a' string. An attempt to decode 'n/a' into float will end up with error, because strconv.ParseFloat expects 'NaN'. Knowing that, we can implement a Map function that will normalize our 'n/a' string and turn it to 'NaN' only for float types.
dec, err := NewDecoder(r)
if err != nil {
log.Fatal(err)
}
dec.Map = func(field, column string, v interface{}) string {
if _, ok := v.(float64); ok && field == "n/a" {
return "NaN"
}
return field
}
Now our float64 fields will be decoded properly into NaN. What about float32, float type aliases and other NaN formats? Look at the full example here.
Different separator/delimiter
Some files may use different value separators, for example TSV files would use \t. The following examples show how to set up a Decoder and Encoder for such use case.
Decoder:
csvReader := csv.NewReader(r)
csvReader.Comma = '\t'
dec, err := NewDecoder(csvReader)
if err != nil {
log.Fatal(err)
}
var users []User
for {
var u User
if err := dec.Decode(&u); err == io.EOF {
break
} else if err != nil {
log.Fatal(err)
}
users = append(users, u)
}
Encoder:
var buf bytes.Buffer
w := csv.NewWriter(&buf)
w.Comma = '\t'
enc := csvutil.NewEncoder(w)
for _, u := range users {
if err := enc.Encode(u); err != nil {
log.Fatal(err)
}
}
w.Flush()
if err := w.Error(); err != nil {
log.Fatal(err)
}
Custom Types and Overrides
There are multiple ways to customize or override your type's behavior.
- a type implements csvutil.Marshaler and/or csvutil.Unmarshaler
type Foo int64
func (f Foo) MarshalCSV() ([]byte, error) {
return strconv.AppendInt(nil, int64(f), 16), nil
}
func (f *Foo) UnmarshalCSV(data []byte) error {
i, err := strconv.ParseInt(string(data), 16, 64)
if err != nil {
return err
}
*f = Foo(i)
return nil
}
- a type implements encoding.TextUnmarshaler and/or encoding.TextMarshaler
type Foo int64
func (f Foo) MarshalText() ([]byte, error) {
return strconv.AppendInt(nil, int64(f), 16), nil
}
func (f *Foo) UnmarshalText(data []byte) error {
i, err := strconv.ParseInt(string(data), 16, 64)
if err != nil {
return err
}
*f = Foo(i)
return nil
}
- a type is registered using Encoder.Register and/or Decoder.Register
type Foo int64
enc.Register(func(f Foo) ([]byte, error) {
return strconv.AppendInt(nil, int64(f), 16), nil
})
dec.Register(func(data []byte, f *Foo) error {
v, err := strconv.ParseInt(string(data), 16, 64)
if err != nil {
return err
}
*f = Foo(v)
return nil
})
- a type implements an interface that was registered using Encoder.Register and/or Decoder.Register
type Foo int64
func (f Foo) String() string {
return strconv.FormatInt(int64(f), 16)
}
func (f *Foo) Scan(state fmt.ScanState, verb rune) error {
// too long; look here: https://github.com/jszwec/csvutil/blob/master/example_decoder_register_test.go#L19
}
enc.Register(func(s fmt.Stringer) ([]byte, error) {
return []byte(s.String()), nil
})
dec.Register(func(data []byte, s fmt.Scanner) error {
_, err := fmt.Sscan(string(data), s)
return err
})
The order of precedence for both Encoder and Decoder is:
- type is registered
- type implements an interface that was registered
- csvutil.{Un,M}arshaler
- encoding.Text{Un,M}arshaler
For more examples look here
Custom time.Time format
Type time.Time can be used as is in the struct fields by both Decoder and Encoder due to the fact that both have builtin support for encoding.TextUnmarshaler and encoding.TextMarshaler. This means that by default Time has a specific format; look at MarshalText and UnmarshalText. There are two ways to override it, which one you choose depends on your use case:
- Via Register func (based on encoding/json)
const format = "2006/01/02 15:04:05"
marshalTime := func(t time.Time) ([]byte, error) {
return t.AppendFormat(nil, format), nil
}
unmarshalTime := func(data []byte, t *time.Time) error {
tt, err := time.Parse(format, string(data))
if err != nil {
return err
}
*t = tt
return nil
}
enc := csvutil.NewEncoder(w)
enc.Register(marshalTime)
dec, err := csvutil.NewDecoder(r)
if err != nil {
return err
}
dec.Register(unmarshalTime)
- With custom type:
type Time struct {
time.Time
}
const format = "2006/01/02 15:04:05"
func (t Time) MarshalCSV() ([]byte, error) {
var b [len(format)]byte
return t.AppendFormat(b[:0], format), nil
}
func (t *Time) UnmarshalCSV(data []byte) error {
tt, err := time.Parse(format, string(data))
if err != nil {
return err
}
*t = Time{Time: tt}
return nil
}
Custom struct tags
Like in other Go encoding packages struct field tags can be used to set custom names or options. By default encoders and decoders are looking at csv tag. However, this can be overriden by manually setting the Tag field.
type Foo struct {
Bar int `custom:"bar"`
}
dec, err := csvutil.NewDecoder(r)
if err != nil {
log.Fatal(err)
}
dec.Tag = "custom"
enc := csvutil.NewEncoder(w)
enc.Tag = "custom"
Slice and Map fields
There is no default encoding/decoding support for slice and map fields because there is no CSV spec for such values. In such case, it is recommended to create a custom type alias and implement Marshaler and Unmarshaler interfaces. Please note that slice and map aliases behave differently than aliases of other types - there is no need for type casting.
type Strings []string
func (s Strings) MarshalCSV() ([]byte, error) {
return []byte(strings.Join(s, ",")), nil // strings.Join takes []string but it will also accept Strings
}
type StringMap map[string]string
func (sm StringMap) MarshalCSV() ([]byte, error) {
return []byte(fmt.Sprint(sm)), nil
}
func main() {
b, err := csvutil.Marshal([]struct {
Strings Strings `csv:"strings"`
Map StringMap `csv:"map"`
}{
{[]string{"a", "b"}, map[string]string{"a": "1"}}, // no type casting is required for slice and map aliases
{Strings{"c", "d"}, StringMap{"b": "1"}},
})
if err != nil {
log.Fatal(err)
}
fmt.Printf("%s\n", b)
// Output:
// strings,map
// "a,b",map[a:1]
// "c,d",map[b:1]
}
Nested/Embedded structs
Both Encoder and Decoder support nested or embedded structs.
Playground: https://play.golang.org/p/ZySjdVkovbf
package main
import (
"fmt"
"github.com/jszwec/csvutil"
)
type Address struct {
Street string `csv:"street"`
City string `csv:"city"`
}
type User struct {
Name string `csv:"name"`
Address
}
func main() {
users := []User{
{
Name: "John",
Address: Address{
Street: "Boylston",
City: "Boston",
},
},
}
b, err := csvutil.Marshal(users)
if err != nil {
panic(err)
}
fmt.Printf("%s\n", b)
var out []User
if err := csvutil.Unmarshal(b, &out); err != nil {
panic(err)
}
fmt.Printf("%+v\n", out)
// Output:
//
// name,street,city
// John,Boylston,Boston
//
// [{Name:John Address:{Street:Boylston City:Boston}}]
}
Inline tag
Fields with inline tag behave similarly to embedded struct fields. However, it gives a possibility to specify the prefix for all underlying fields. This can be useful when one structure can define multiple CSV columns because they are different from each other only by a certain prefix. Look at the example below.
Playground: https://play.golang.org/p/jyEzeskSnj7
package main
import (
"fmt"
"github.com/jszwec/csvutil"
)
func main() {
type Address struct {
Street string `csv:"street"`
City string `csv:"city"`
}
type User struct {
Name string `csv:"name"`
Address Address `csv:",inline"`
HomeAddress Address `csv:"home_address_,inline"`
WorkAddress Address `csv:"work_address_,inline"`
Age int `csv:"age,omitempty"`
}
users := []User{
{
Name: "John",
Address: Address{"Washington", "Boston"},
HomeAddress: Address{"Boylston", "Boston"},
WorkAddress: Address{"River St", "Cambridge"},
Age: 26,
},
}
b, err := csvutil.Marshal(users)
if err != nil {
fmt.Println("error:", err)
}
fmt.Printf("%s\n", b)
// Output:
// name,street,city,home_address_street,home_address_city,work_address_street,work_address_city,age
// John,Washington,Boston,Boylston,Boston,River St,Cambridge,26
}
Performance
csvutil provides the best encoding and decoding performance with small memory usage.
Unmarshal
csvutil:
BenchmarkUnmarshal/csvutil.Unmarshal/1_record-12 280696 4516 ns/op 7332 B/op 26 allocs/op
BenchmarkUnmarshal/csvutil.Unmarshal/10_records-12 95750 11517 ns/op 8356 B/op 35 allocs/op
BenchmarkUnmarshal/csvutil.Unmarshal/100_records-12 14997 83146 ns/op 18532 B/op 125 allocs/op
BenchmarkUnmarshal/csvutil.Unmarshal/1000_records-12 1485 750143 ns/op 121094 B/op 1025 allocs/op
BenchmarkUnmarshal/csvutil.Unmarshal/10000_records-12 154 7587205 ns/op 1136662 B/op 10025 allocs/op
BenchmarkUnmarshal/csvutil.Unmarshal/100000_records-12 14 76126616 ns/op 11808744 B/op 100025 allocs/op
gocsv:
BenchmarkUnmarshal/gocsv.Unmarshal/1_record-12 141330 7499 ns/op 7795 B/op 97 allocs/op
BenchmarkUnmarshal/gocsv.Unmarshal/10_records-12 54252 21664 ns/op 13891 B/op 307 allocs/op
BenchmarkUnmarshal/gocsv.Unmarshal/100_records-12 6920 159662 ns/op 72644 B/op 2380 allocs/op
BenchmarkUnmarshal/gocsv.Unmarshal/1000_records-12 752 1556083 ns/op 650248 B/op 23083 allocs/op
BenchmarkUnmarshal/gocsv.Unmarshal/10000_records-12 72 17086623 ns/op 7017469 B/op 230092 allocs/op
BenchmarkUnmarshal/gocsv.Unmarshal/100000_records-12 7 163610749 ns/op 75004923 B/op 2300105 allocs/op
easycsv:
BenchmarkUnmarshal/easycsv.ReadAll/1_record-12 101527 10662 ns/op 8855 B/op 81 allocs/op
BenchmarkUnmarshal/easycsv.ReadAll/10_records-12 23325 51437 ns/op 24072 B/op 391 allocs/op
BenchmarkUnmarshal/easycsv.ReadAll/100_records-12 2402 447296 ns/op 170538 B/op 3454 allocs/op
BenchmarkUnmarshal/easycsv.ReadAll/1000_records-12 272 4370854 ns/op 1595683 B/op 34057 allocs/op
BenchmarkUnmarshal/easycsv.ReadAll/10000_records-12 24 47502457 ns/op 18861808 B/op 340068 allocs/op
BenchmarkUnmarshal/easycsv.ReadAll/100000_records-12 3 468974170 ns/op 189427066 B/op 3400082 allocs/op
Marshal
csvutil:
BenchmarkMarshal/csvutil.Marshal/1_record-12 279558 4390 ns/op 9952 B/op 12 allocs/op
BenchmarkMarshal/csvutil.Marshal/10_records-12 82478 15608 ns/op 10800 B/op 21 allocs/op
BenchmarkMarshal/csvutil.Marshal/100_records-12 10275 117288 ns/op 28208 B/op 112 allocs/op
BenchmarkMarshal/csvutil.Marshal/1000_records-12 1075 1147473 ns/op 168508 B/op 1014 allocs/op
BenchmarkMarshal/csvutil.Marshal/10000_records-12 100 11985382 ns/op 1525973 B/op 10017 allocs/op
BenchmarkMarshal/csvutil.Marshal/100000_records-12 9 113640813 ns/op 22455873 B/op 100021 allocs/op
gocsv:
BenchmarkMarshal/gocsv.Marshal/1_record-12 203052 6077 ns/op 5914 B/op 81 allocs/op
BenchmarkMarshal/gocsv.Marshal/10_records-12 50132 24585 ns/op 9284 B/op 360 allocs/op
BenchmarkMarshal/gocsv.Marshal/100_records-12 5480 212008 ns/op 51916 B/op 3151 allocs/op
BenchmarkMarshal/gocsv.Marshal/1000_records-12 514 2053919 ns/op 444506 B/op 31053 allocs/op
BenchmarkMarshal/gocsv.Marshal/10000_records-12 52 21066666 ns/op 4332377 B/op 310064 allocs/op
BenchmarkMarshal/gocsv.Marshal/100000_records-12 5 207408929 ns/op 51169419 B/op 3100077 allocs/op
![csvutil: unsupported type: []string](https://avatars.githubusercontent.com/u/50327099?v=4)
