Golang: deepcopy map[string]interface{}. Could be used for any other Go type with minor modifications.

deepcopy.go

// Package deepcopy provides a function for deep copying map[string]interface{}
// values. Inspired by the StackOverflow answer at:
// http://stackoverflow.com/a/28579297/1366283
//
// Uses the golang.org/pkg/encoding/gob package to do this and therefore has the
// same caveats.
// See: https://blog.golang.org/gobs-of-data
// See: https://golang.org/pkg/encoding/gob/
package deepcopy

import (
	"bytes"
	"encoding/gob"
)

func init() {
	gob.Register(map[string]interface{}{})
}

// Map performs a deep copy of the given map m.
func Map(m map[string]interface{}) (map[string]interface{}, error) {
	var buf bytes.Buffer
	enc := gob.NewEncoder(&buf)
	dec := gob.NewDecoder(&buf)
	err := enc.Encode(m)
	if err != nil {
		return nil, err
	}
	var copy map[string]interface{}
	err = dec.Decode(&copy)
	if err != nil {
		return nil, err
	}
	return copy, nil
}

deepcopy_test.go

package deepcopy

import (
	"testing"

	"github.com/stretchr/testify/assert"
)

func TestMap(t *testing.T) {
	testCases := []struct {
		// original and expectedOriginal are the same value in each test case. We do
		// this to avoid unintentionally asserting against a mutated
		// expectedOriginal and having the test pass erroneously. We also do not
		// want to rely on the deep copy function we are testing to ensure this does
		// not happen.
		original         map[string]interface{}
		transformer      func(m map[string]interface{}) map[string]interface{}
		expectedCopy     map[string]interface{}
		expectedOriginal map[string]interface{}
	}{
		// reassignment of entire map, should be okay even without deepcopy.
		{
			original: nil,
			transformer: func(m map[string]interface{}) map[string]interface{} {
				return map[string]interface{}{}
			},
			expectedCopy:     map[string]interface{}{},
			expectedOriginal: nil,
		},
		{
			original: map[string]interface{}{},
			transformer: func(m map[string]interface{}) map[string]interface{} {
				return nil
			},
			expectedCopy:     nil,
			expectedOriginal: map[string]interface{}{},
		},
		// mutation of map
		{
			original: map[string]interface{}{},
			transformer: func(m map[string]interface{}) map[string]interface{} {
				m["foo"] = "bar"
				return m
			},
			expectedCopy: map[string]interface{}{
				"foo": "bar",
			},
			expectedOriginal: map[string]interface{}{},
		},
		{
			original: map[string]interface{}{
				"foo": "bar",
			},
			transformer: func(m map[string]interface{}) map[string]interface{} {
				m["foo"] = "car"
				return m
			},
			expectedCopy: map[string]interface{}{
				"foo": "car",
			},
			expectedOriginal: map[string]interface{}{
				"foo": "bar",
			},
		},
		// mutation of nested maps
		{
			original: map[string]interface{}{},
			transformer: func(m map[string]interface{}) map[string]interface{} {
				m["foo"] = map[string]interface{}{
					"biz": "baz",
				}
				return m
			},
			expectedCopy: map[string]interface{}{
				"foo": map[string]interface{}{
					"biz": "baz",
				},
			},
			expectedOriginal: map[string]interface{}{},
		},
		{
			original: map[string]interface{}{
				"foo": map[string]interface{}{
					"biz": "booz",
					"gaz": "gooz",
				},
			},
			transformer: func(m map[string]interface{}) map[string]interface{} {
				m["foo"] = map[string]interface{}{
					"biz": "baz",
				}
				return m
			},
			expectedCopy: map[string]interface{}{
				"foo": map[string]interface{}{
					"biz": "baz",
				},
			},
			expectedOriginal: map[string]interface{}{
				"foo": map[string]interface{}{
					"biz": "booz",
					"gaz": "gooz",
				},
			},
		},
		// mutation of slice values
		{
			original: map[string]interface{}{
				"foo": []string{"biz", "baz"},
			},
			transformer: func(m map[string]interface{}) map[string]interface{} {
				m["foo"].([]string)[0] = "hiz"
				return m
			},
			expectedCopy: map[string]interface{}{
				"foo": []string{"hiz", "baz"},
			},
			expectedOriginal: map[string]interface{}{
				"foo": []string{"biz", "baz"},
			},
		},
	}
	for i, tc := range testCases {
		copy, err := Map(tc.original)
		assert.NoError(t, err)
		assert.Exactly(t, tc.expectedCopy, tc.transformer(copy), "copy was not mutated. test case: %d", i)
		assert.Exactly(t, tc.expectedOriginal, tc.original, "original was mutated. test case: %d", i)
	}
}