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attribute: add String method to Value and KeyValue #7812

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1 change: 1 addition & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
- Add `go.opentelemetry.io/otel/semconv/v1.39.0` package.
The package contains semantic conventions from the `v1.39.0` version of the OpenTelemetry Semantic Conventions.
See the [migration documentation](./semconv/v1.39.0/MIGRATION.md) for information on how to upgrade from `go.opentelemetry.io/otel/semconv/v1.38.0.`(#7783)
- Add `String` method to `attribute.Value` and `KeyValue`. (#7812)
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@pellared pellared Jan 20, 2026

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Suggested change
- Add `String` method to `attribute.Value` and `KeyValue`. (#7812)
- Add `String` method to `attribute.Value` and `KeyValue` in `go.opentelemetry.io/otel/attribute`. (#7812)


### Changed

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5 changes: 5 additions & 0 deletions attribute/kv.go
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Expand Up @@ -18,6 +18,11 @@ func (kv KeyValue) Valid() bool {
return kv.Key.Defined() && kv.Value.Type() != INVALID
}

// String implements the Stringer interface, used when you pass an object to fmt.Println, etc.
func (kv KeyValue) String() string {
return "{" + string(kv.Key) + ":" + kv.Value.String() + "}"
}
Comment on lines +21 to +24
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@pellared pellared Jan 20, 2026

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Suggested change
// String implements the Stringer interface, used when you pass an object to fmt.Println, etc.
func (kv KeyValue) String() string {
return "{" + string(kv.Key) + ":" + kv.Value.String() + "}"
}
// String returns key-value pair as a string, formatted like "key:value".
//
// The returned string is meant for debugging;
// the string representation is not stable.
func (kv KeyValue) String() string {
return string(kv.Key) + ":" + kv.Value.String()
}

Why? To follow prior art https://pkg.go.dev/go.opentelemetry.io/otel/log#KeyValue.String (more: #5117).


// Bool creates a KeyValue with a BOOL Value type.
func Bool(k string, v bool) KeyValue {
return Key(k).Bool(v)
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81 changes: 81 additions & 0 deletions attribute/kv_test.go
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Expand Up @@ -169,3 +169,84 @@ func TestIncorrectCast(t *testing.T) {
})
}
}

func TestKeyValueString(t *testing.T) {
tests := []struct {
name string
kv attribute.KeyValue
want string
}{
{
name: "int positive",
kv: attribute.Int("key", 42),
want: "{key:42}",
},
{
name: "float64 negative",
kv: attribute.Float64("key", -3.14),
want: "{key:-3.14}",
},
{
name: "string simple",
kv: attribute.String("key", "value"),
want: "{key:value}",
},
{
name: "string empty",
kv: attribute.String("key", ""),
want: "{key:}",
},
{
name: "string with spaces",
kv: attribute.String("key", "hello world"),
want: "{key:hello world}",
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@bboreham bboreham Jan 20, 2026

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I find it surprising that strings are not quoted, especially with embedded spaces like here.
But this is what Value.Emit() did, so leaving this here for comment.

},
{
name: "bool slice",
kv: attribute.BoolSlice("key", []bool{true, false, true}),
want: "{key:[true false true]}",
},
{
name: "int slice",
kv: attribute.IntSlice("key", []int{1, 2, 3}),
want: "{key:[1,2,3]}",
},
{
name: "int64 slice",
kv: attribute.Int64Slice("key", []int64{1, 2, 3}),
want: "{key:[1,2,3]}",
},
{
name: "float64 slice",
kv: attribute.Float64Slice("key", []float64{1.5, 2.5, 3.5}),
want: "{key:[1.5,2.5,3.5]}",
},
{
name: "string slice",
kv: attribute.StringSlice("key", []string{"foo", "bar"}),
want: `{key:["foo","bar"]}`,
},
{
name: "empty key",
kv: attribute.String("", "value"),
want: "{:value}",
},
{
name: "invalid/uninitialized KeyValue",
kv: attribute.KeyValue{},
want: "{:unknown}",
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@bboreham bboreham Jan 20, 2026

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Maybe this should print something clearer?
Originally I wanted this for debugging purposes, so exactly what is in the object may be important.

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@pellared pellared Jan 20, 2026

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I propose <invalid>.

},
{
name: "key with special characters",
kv: attribute.String("key-with-dashes", "value"),
want: "{key-with-dashes:value}",
},
}

for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
got := tt.kv.String()
assert.Equal(t, tt.want, got)
})
}
}
6 changes: 6 additions & 0 deletions attribute/value.go
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Expand Up @@ -222,6 +222,12 @@ func (v Value) AsInterface() any {
return unknownValueType{}
}

// String implements the Stringer interface, used when you pass an object to fmt.Println, etc.
// Do not confuse with AsString, which you should call if you know the value is of String type.
Comment on lines +225 to +226
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@pellared pellared Jan 20, 2026

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Suggested change
// String implements the Stringer interface, used when you pass an object to fmt.Println, etc.
// Do not confuse with AsString, which you should call if you know the value is of String type.
// String returns Value's value as a string, formatted like [fmt.Sprint].
//
// The returned string is meant for debugging;
// the string representation is not stable.

func (v Value) String() string {
return v.Emit()
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@pellared pellared Jan 20, 2026

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Shouldn't we deprecate Emit so that users migrate to the new method?

}

// Emit returns a string representation of Value's data.
func (v Value) Emit() string {
switch v.Type() {
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101 changes: 101 additions & 0 deletions attribute/value_test.go
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Expand Up @@ -206,3 +206,104 @@ func TestAsSlice(t *testing.T) {
ss2 := kv.Value.AsStringSlice()
assert.Equal(t, ss1, ss2)
}

func TestValueString(t *testing.T) {
tests := []struct {
name string
value attribute.Value
want string
}{
{
name: "bool true",
value: attribute.BoolValue(true),
want: "true",
},
{
name: "int64 positive",
value: attribute.Int64Value(42),
want: "42",
},
{
name: "int negative",
value: attribute.IntValue(-42),
want: "-42",
},
{
name: "float64 positive",
value: attribute.Float64Value(42.5),
want: "42.5",
},
{
name: "float64 scientific notation",
value: attribute.Float64Value(1.23e-10),
want: "1.23e-10",
},
{
name: "string empty",
value: attribute.StringValue(""),
want: "",
},
{
name: "string with spaces",
value: attribute.StringValue("hello world"),
want: "hello world",
},
{
name: "string with special characters",
value: attribute.StringValue("hello\nworld\t!"),
want: "hello\nworld\t!",
},
{
name: "bool slice",
value: attribute.BoolSliceValue([]bool{true, false, true}),
want: "[true false true]",
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@bboreham bboreham Jan 20, 2026

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Also it was surprising to me that this one has no commas, while the int, float and string slices do.

},
{
name: "bool slice empty",
value: attribute.BoolSliceValue([]bool{}),
want: "[]",
},
{
name: "int64 slice",
value: attribute.Int64SliceValue([]int64{1, -2, 3}),
want: "[1,-2,3]",
},
{
name: "int64 slice empty",
value: attribute.Int64SliceValue([]int64{}),
want: "[]",
},
{
name: "int slice",
value: attribute.IntSliceValue([]int{1, -2, 3}),
want: "[1,-2,3]",
},
{
name: "float64 slice",
value: attribute.Float64SliceValue([]float64{1.5, -2.5, 3.0}),
want: "[1.5,-2.5,3]",
},
{
name: "string slice",
value: attribute.StringSliceValue([]string{"foo", "bar", "baz"}),
want: `["foo","bar","baz"]`,
},
{
name: "string slice with empty strings",
value: attribute.StringSliceValue([]string{"", "bar", ""}),
want: `["","bar",""]`,
},
{
name: "invalid type",
value: attribute.Value{},
want: "unknown",
},
}

for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
got := tt.value.String()
assert.Equal(t, tt.want, got)
})
}
}