improve compatibility with go/flag

go/flag in go 1.13 has a public Output() function and a Name() function
- change out() to Output() (adjusting the rest of the file)
- add Name() and tests for it

From: @mckern (spf13#220) re: out/Output:
This brings behavior inline with go's flag library, and allows for
printing output directly to whatever the current FlagSet is using for
output. This change will make it easier to correctly emit output to
stdout or stderr (e.g. a user has requested a help screen, which
should emit to stdout since it's the desired outcome).

Author: Chloe Kudryavtsev

flag.go

@@ -160,7 +160,7 @@ type FlagSet struct {
 	args              []string // arguments after flags
 	argsLenAtDash     int      // len(args) when a '--' was located when parsing, or -1 if no --
 	errorHandling     ErrorHandling
-	output            io.Writer // nil means stderr; use out() accessor
+	output            io.Writer // nil means stderr; use Output() accessor
 	interspersed      bool      // allow interspersed option/non-option args
 	normalizeNameFunc func(f *FlagSet, name string) NormalizedName
 
@@ -255,13 +255,20 @@ func (f *FlagSet) normalizeFlagName(name string) NormalizedName {
 	return n(f, name)
 }
 
-func (f *FlagSet) out() io.Writer {
+// Output returns the destination for usage and error messages. os.Stderr is returned if
+// output was not set or was set to nil.
+func (f *FlagSet) Output() io.Writer {
 	if f.output == nil {
 		return os.Stderr
 	}
 	return f.output
 }
 
+// Name returns the name of the flag set.
+func (f *FlagSet) Name() string {
+	return f.name
+}
+
 // SetOutput sets the destination for usage and error messages.
 // If output is nil, os.Stderr is used.
 func (f *FlagSet) SetOutput(output io.Writer) {
@@ -358,7 +365,7 @@ func (f *FlagSet) ShorthandLookup(name string) *Flag {
 	}
 	if len(name) > 1 {
 		msg := fmt.Sprintf("can not look up shorthand which is more than one ASCII character: %q", name)
-		fmt.Fprintf(f.out(), msg)
+		fmt.Fprintf(f.Output(), msg)
 		panic(msg)
 	}
 	c := name[0]
@@ -482,7 +489,7 @@ func (f *FlagSet) Set(name, value string) error {
 	}
 
 	if flag.Deprecated != "" {
-		fmt.Fprintf(f.out(), "Flag --%s has been deprecated, %s\n", flag.Name, flag.Deprecated)
+		fmt.Fprintf(f.Output(), "Flag --%s has been deprecated, %s\n", flag.Name, flag.Deprecated)
 	}
 	return nil
 }
@@ -523,7 +530,7 @@ func Set(name, value string) error {
 // otherwise, the default values of all defined flags in the set.
 func (f *FlagSet) PrintDefaults() {
 	usages := f.FlagUsages()
-	fmt.Fprint(f.out(), usages)
+	fmt.Fprint(f.Output(), usages)
 }
 
 // defaultIsZeroValue returns true if the default value for this flag represents
@@ -758,7 +765,7 @@ func PrintDefaults() {
 
 // defaultUsage is the default function to print a usage message.
 func defaultUsage(f *FlagSet) {
-	fmt.Fprintf(f.out(), "Usage of %s:\n", f.name)
+	fmt.Fprintf(f.Output(), "Usage of %s:\n", f.name)
 	f.PrintDefaults()
 }
 
@@ -844,7 +851,7 @@ func (f *FlagSet) AddFlag(flag *Flag) {
 	_, alreadyThere := f.formal[normalizedFlagName]
 	if alreadyThere {
 		msg := fmt.Sprintf("%s flag redefined: %s", f.name, flag.Name)
-		fmt.Fprintln(f.out(), msg)
+		fmt.Fprintln(f.Output(), msg)
 		panic(msg) // Happens only if flags are declared with identical names
 	}
 	if f.formal == nil {
@@ -860,7 +867,7 @@ func (f *FlagSet) AddFlag(flag *Flag) {
 	}
 	if len(flag.Shorthand) > 1 {
 		msg := fmt.Sprintf("%q shorthand is more than one ASCII character", flag.Shorthand)
-		fmt.Fprintf(f.out(), msg)
+		fmt.Fprintf(f.Output(), msg)
 		panic(msg)
 	}
 	if f.shorthands == nil {
@@ -870,7 +877,7 @@ func (f *FlagSet) AddFlag(flag *Flag) {
 	used, alreadyThere := f.shorthands[c]
 	if alreadyThere {
 		msg := fmt.Sprintf("unable to redefine %q shorthand in %q flagset: it's already used for %q flag", c, f.name, used.Name)
-		fmt.Fprintf(f.out(), msg)
+		fmt.Fprintf(f.Output(), msg)
 		panic(msg)
 	}
 	f.shorthands[c] = flag
@@ -909,7 +916,7 @@ func VarP(value Value, name, shorthand, usage string) {
 func (f *FlagSet) failf(format string, a ...interface{}) error {
 	err := fmt.Errorf(format, a...)
 	if f.errorHandling != ContinueOnError {
-		fmt.Fprintln(f.out(), err)
+		fmt.Fprintln(f.Output(), err)
 		f.usage()
 	}
 	return err
@@ -1060,7 +1067,7 @@ func (f *FlagSet) parseSingleShortArg(shorthands string, args []string, fn parse
 	}
 
 	if flag.ShorthandDeprecated != "" {
-		fmt.Fprintf(f.out(), "Flag shorthand -%s has been deprecated, %s\n", flag.Shorthand, flag.ShorthandDeprecated)
+		fmt.Fprintf(f.Output(), "Flag shorthand -%s has been deprecated, %s\n", flag.Shorthand, flag.ShorthandDeprecated)
 	}
 
 	err = fn(flag, value)

flag_test.go

@@ -115,6 +115,13 @@ func TestAddFlagSet(t *testing.T) {
 	oldSet := NewFlagSet("old", ContinueOnError)
 	newSet := NewFlagSet("new", ContinueOnError)
 
+	if oldSet.Name() != "old" {
+		t.Errorf("When reading a flagset's name, expected %s, but found %s", "old", oldSet.Name())
+	}
+	if newSet.Name() != "new" {
+		t.Errorf("When reading a flagset's name, expected %s, but found %s", "new", newSet.Name())
+	}
+
 	oldSet.String("flag1", "flag1", "flag1")
 	oldSet.String("flag2", "flag2", "flag2")