Leveled fields: Fields present only at certain log levels

example_test.go

@@ -358,3 +358,23 @@ func ExampleWrapCore_wrap() {
 	// {"level":"info","msg":"doubled"}
 	// {"level":"info","msg":"doubled"}
 }
+
+func ExampleDebugField() {
+	logger := zap.NewExample()
+	defer logger.Sync()
+
+	logger.Info(".Info call filters out the debug field",
+		zap.Int("IntKey", 1),
+		zap.DebugField(zap.String("Key", "value")),
+		zap.Duration("DurationKey", time.Second),
+	)
+	logger.Debug(".Debug call does NOT filter out the debug field",
+		zap.Int("IntKey", 1),
+		zap.DebugField(zap.String("Key", "value")),
+		zap.Duration("DurationKey", time.Second),
+	)
+
+	// Output:
+	// {"level":"info","msg":".Info call filters out the debug field","IntKey":1,"DurationKey":"1s"}
+	// {"level":"debug","msg":".Debug call does NOT filter out the debug field","IntKey":1,"Key":"value","DurationKey":"1s"}
+}

field.go

@@ -35,8 +35,17 @@ type Field = zapcore.Field
 var (
 	_minTimeInt64 = time.Unix(0, math.MinInt64)
 	_maxTimeInt64 = time.Unix(0, math.MaxInt64)
+
+	debugLevelField = zapcore.DebugLevel
 )
 
+// DebugField wraps a field so that DebugLevel log displays it.
+// See https://github.com/uber-go/zap/issues/1078 for motivation.
+func DebugField(f Field) Field {
+	f.Level = &debugLevelField
+	return f
+}
+
 // Skip constructs a no-op field, which is often useful when handling invalid
 // inputs in other Field constructors.
 func Skip() Field {

zapcore/entry.go

@@ -249,8 +249,19 @@ func (ce *CheckedEntry) Write(fields ...Field) {
 	ce.dirty = true
 
 	var err error
+
+	// https://github.com/golang/go/wiki/SliceTricks#filtering-without-allocating
+	filteredFields := fields[:0]
+	for _, field := range fields {
+		// include the field if the level is unset or
+		// the field level is higher than the entry's level.
+		if field.Level == nil || *field.Level >= ce.Entry.Level {
+			filteredFields = append(filteredFields, field)
+		}
+	}
+
 	for i := range ce.cores {
-		err = multierr.Append(err, ce.cores[i].Write(ce.Entry, fields))
+		err = multierr.Append(err, ce.cores[i].Write(ce.Entry, filteredFields))
 	}
 	if err != nil && ce.ErrorOutput != nil {
 		fmt.Fprintf(ce.ErrorOutput, "%v write error: %v\n", ce.Time, err)
@@ -259,7 +270,7 @@ func (ce *CheckedEntry) Write(fields ...Field) {
 
 	hook := ce.after
 	if hook != nil {
-		hook.OnWrite(ce, fields)
+		hook.OnWrite(ce, filteredFields)
 	}
 	putCheckedEntry(ce)
 }

zapcore/field.go

@@ -106,6 +106,7 @@ type Field struct {
 	Type      FieldType
 	Integer   int64
 	String    string
+	Level     *Level
 	Interface interface{}
 }