Nested data structures in Go - Python equivalent

Question

I can create this data structure in Python really easily:

data = {'Locations': [],
 'Dates': [],
 'Properties': [{'key': 'data1', 'value': 'data2'}],
 'Category': 'all'}

Which can then be marshalled to JSON in Python just as easily. e.g.

print json.dumps(data)

{"Category": "all", "Dates": [], "Locations": [], "Properties": [{"value": "data2", "key": "data1"}]}

However, I'm tearing my hair out trying to create the same structure then convert it to JSON in Go. Go looks to be very promising and just what I need for creating cross platform applications, but boy this stuff seems to be frustratingly difficult.

This is what I've tried, however I cant get the structure to include the square brackets that should surround the properties element.

import (
"fmt"
"encoding/json"
)

func main() {

data := map[string]interface{}{"Offset": "0", "Properties": map[string]string{"value": "data2", "key": "data1"}, "Category": "all", "Locations": []string{}, "Dates": []string{} }
    data_json, _ := json.Marshal(data)
fmt.Println(string(data_json))
}

Which outputs:

{"Category":"all","Dates":[],"Locations":[],"Offset":"0","Properties":{"key":"data1","value":"data2"}}

Heres a demo: http://play.golang.org/p/49Kytg6v_C

OneOfOne · Accepted Answer · 2014-12-18 00:03:21Z

2

You just need to create a slice of map[string]string:

data := map[string]interface{}{
    "Offset":     "0",
    "Properties": []map[string]string{{"value": "data2", "key": "data1"}},
    "Category":   "all",
    "Locations":  []string{},
    "Dates":      []string{},
}

playground

answered Dec 18, 2014 at 0:03

OneOfOne

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2 Comments

user1513388 Over a year ago

Thank you so much - I've been going round in circles with this for hours.

David Budworth Over a year ago

If the type info is making it too noisy (ie: for more complicated structures), don't forget you can define types to simply the map literal syntax. Example: play.golang.org/p/FCjaDqdIUw

Linear · Accepted Answer · 2014-12-18 02:10:08Z

While OneOfOne's solution works in a literal sense, with Go's static typing you likely want a struct.

// The `json:"stuff"` will cause it to use that json tag when reading or writing json.
type Property struct {
    Key string `json:"key"`
    Val string `json:"value"`
}

// ,omitempty allows it to ignore the empty string "" when writing json,
// I put this here because one of your json examples had no Offset field
// you can do this with other fields too.
type MyType struct {
    Offset     string `json:",omitempty"`
    Properties []Property
    Category   string
    Locations  []string
    Dates      []string
}

Of course, you could also consider using built-in or custom Go types for some of those fields, such as using a []time.Time for the Dates field. This makes it more difficult to just read/write arbitrary json ad-hoc or on the fly, but since you need to have some logic somewhere to interpret those fields, in Go it generally makes much more sense to treat it as a struct most of the time.

To read in/put out the json, you would then do

import "encoding/json"
//...
stuff := MyType{}
json.Unmarshal(myJsonData, &stuff)
// stuff is now { [{data1 data2}] all [] []}
out,_ := json.Marshal(stuff)
// string(out) is now {"Properties":[{"key":"data1","value":"data2"}],
// "Category":"all","Locations":[],"Dates":[]}

Playground: http://play.golang.org/p/jIHgXmY13R

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