Add config option AlwaysIncludeTrailingComma, which forces the writing of commas

in arrays/slices/maps. Useful in tests to avoid false positives when diffing.

spew/config.go

@@ -76,6 +76,11 @@ type ConfigState struct {
 	// data structures in tests.
 	DisableCapacities bool
 
+	// AlwaysIncludeTrailingComma specifies whether to always include a trailing
+	// comma, Go-style. This is useful to avoid false positives when diffing data
+	// structures in tests.
+	AlwaysIncludeTrailingComma bool
+
 	// ContinueOnMethod specifies whether or not recursion should continue once
 	// a custom error or Stringer interface is invoked.  The default, false,
 	// means it will print the results of invoking the custom error or Stringer

spew/dump.go

@@ -236,11 +236,7 @@ func (d *dumpState) dumpSlice(v reflect.Value) {
 	// Recursively call dump for each item.
 	for i := 0; i < numEntries; i++ {
 		d.dump(d.unpackValue(v.Index(i)))
-		if i < (numEntries - 1) {
-			d.w.Write(commaNewlineBytes)
-		} else {
-			d.w.Write(newlineBytes)
-		}
+		d.writeComma(i < (numEntries - 1))
 	}
 }
 
@@ -392,11 +388,7 @@ func (d *dumpState) dump(v reflect.Value) {
 				d.w.Write(colonSpaceBytes)
 				d.ignoreNextIndent = true
 				d.dump(d.unpackValue(v.MapIndex(key)))
-				if i < (numEntries - 1) {
-					d.w.Write(commaNewlineBytes)
-				} else {
-					d.w.Write(newlineBytes)
-				}
+				d.writeComma(i < (numEntries - 1))
 			}
 		}
 		d.depth--
@@ -419,11 +411,7 @@ func (d *dumpState) dump(v reflect.Value) {
 				d.w.Write(colonSpaceBytes)
 				d.ignoreNextIndent = true
 				d.dump(d.unpackValue(v.Field(i)))
-				if i < (numFields - 1) {
-					d.w.Write(commaNewlineBytes)
-				} else {
-					d.w.Write(newlineBytes)
-				}
+				d.writeComma(i < (numFields - 1))
 			}
 		}
 		d.depth--
@@ -448,6 +436,16 @@ func (d *dumpState) dump(v reflect.Value) {
 	}
 }
 
+// writeComma emits a comma if hasMoreElements is true, or if trailing commas
+// are always enabled.
+func (d *dumpState) writeComma(hasMoreElements bool) {
+	if hasMoreElements || d.cs.AlwaysIncludeTrailingComma {
+		d.w.Write(commaNewlineBytes)
+	} else {
+		d.w.Write(newlineBytes)
+	}
+}
+
 // fdump is a helper function to consolidate the logic from the various public
 // methods which take varying writers and config states.
 func fdump(cs *ConfigState, w io.Writer, a ...interface{}) {

spew/spew_test.go

@@ -132,6 +132,7 @@ func initSpewTests() {
 	scsContinue := &spew.ConfigState{Indent: " ", ContinueOnMethod: true}
 	scsNoPtrAddr := &spew.ConfigState{DisablePointerAddresses: true}
 	scsNoCap := &spew.ConfigState{DisableCapacities: true}
+	scsTrailingComma := &spew.ConfigState{Indent: " ", AlwaysIncludeTrailingComma: true}
 
 	// Variables for tests on types which implement Stringer interface with and
 	// without a pointer receiver.
@@ -154,6 +155,12 @@ func initSpewTests() {
 	dt := depthTester{indirCir1{nil}, [1]string{"arr"}, []string{"slice"},
 		map[string]int{"one": 1}}
 
+	// commatester is to test trailing commas
+	type commaTester struct {
+		slice []interface{}
+		m     map[string]int
+	}
+
 	// Variable for tests on types which implement error interface.
 	te := customError(10)
 
@@ -203,6 +210,22 @@ func initSpewTests() {
 		{scsNoPtrAddr, fCSSdump, "", tptr, "(*spew_test.ptrTester)({\ns: (*struct {})({\n})\n})\n"},
 		{scsNoCap, fCSSdump, "", make([]string, 0, 10), "([]string) {\n}\n"},
 		{scsNoCap, fCSSdump, "", make([]string, 1, 10), "([]string) (len=1) {\n(string) \"\"\n}\n"},
+		{scsTrailingComma, fCSFdump, "", commaTester{
+			slice: []interface{}{
+				map[string]int{"one": 1},
+			},
+			m: map[string]int{"one": 1},
+		},
+			"(spew_test.commaTester) {\n" +
+				" slice: ([]interface {}) (len=1 cap=1) {\n" +
+				"  (map[string]int) (len=1) {\n" +
+				"   (string) (len=3) \"one\": (int) 1,\n" +
+				"  },\n" +
+				" },\n" +
+				" m: (map[string]int) (len=1) {\n" +
+				"  (string) (len=3) \"one\": (int) 1,\n" +
+				" },\n" +
+				"}\n"},
 	}
 }