store default values during NewParser

parse.go

@@ -1,6 +1,7 @@
 package arg
 
 import (
+	"encoding"
 	"encoding/csv"
 	"errors"
 	"fmt"
@@ -193,6 +194,22 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 		if err != nil {
 			return nil, err
 		}
+
+		// add nonzero field values as defaults
+		for _, spec := range cmd.specs {
+			if v := p.val(spec.dest); v.IsValid() && !isZero(v) {
+				if defaultVal, ok := v.Interface().(encoding.TextMarshaler); ok {
+					str, err := defaultVal.MarshalText()
+					if err != nil {
+						return nil, fmt.Errorf("%v: error marshaling default value to string: %w", spec.dest, err)
+					}
+					spec.defaultVal = string(str)
+				} else {
+					spec.defaultVal = fmt.Sprintf("%v", v)
+				}
+			}
+		}
+
 		p.cmd.specs = append(p.cmd.specs, cmd.specs...)
 		p.cmd.subcommands = append(p.cmd.subcommands, cmd.subcommands...)
 
@@ -706,3 +723,16 @@ func findSubcommand(cmds []*command, name string) *command {
 	}
 	return nil
 }
+
+// isZero returns true if v contains the zero value for its type
+func isZero(v reflect.Value) bool {
+	t := v.Type()
+	if t.Kind() == reflect.Slice {
+		return v.IsNil()
+	}
+	if !t.Comparable() {
+		return false
+	}
+	z := reflect.Zero(t)
+	return v.Interface() == z.Interface()
+}

parse_test.go

@@ -1083,6 +1083,15 @@ func TestDefaultOptionValues(t *testing.T) {
 	assert.True(t, args.G)
 }
 
+func TestDefaultUnparseable(t *testing.T) {
+	var args struct {
+		A int `default:"x"`
+	}
+
+	err := parse("", &args)
+	assert.EqualError(t, err, `error processing default value for --a: strconv.ParseInt: parsing "x": invalid syntax`)
+}
+
 func TestDefaultPositionalValues(t *testing.T) {
 	var args struct {
 		A int      `arg:"positional" default:"123"`

usage.go

@@ -1,11 +1,9 @@
 package arg
 
 import (
-	"encoding"
 	"fmt"
 	"io"
 	"os"
-	"reflect"
 	"strings"
 )
 
@@ -94,7 +92,7 @@ func (p *Parser) writeUsageForCommand(w io.Writer, cmd *command) {
 	fmt.Fprint(w, "\n")
 }
 
-func printTwoCols(w io.Writer, left, help string, defaultVal *string) {
+func printTwoCols(w io.Writer, left, help string, defaultVal string) {
 	lhs := "  " + left
 	fmt.Fprint(w, lhs)
 	if help != "" {
@@ -105,8 +103,8 @@ func printTwoCols(w io.Writer, left, help string, defaultVal *string) {
 		}
 		fmt.Fprint(w, help)
 	}
-	if defaultVal != nil {
-		fmt.Fprintf(w, " [default: %s]", *defaultVal)
+	if defaultVal != "" {
+		fmt.Fprintf(w, " [default: %s]", defaultVal)
 	}
 	fmt.Fprint(w, "\n")
 }
@@ -136,7 +134,7 @@ func (p *Parser) writeHelpForCommand(w io.Writer, cmd *command) {
 	if len(positionals) > 0 {
 		fmt.Fprint(w, "\nPositional arguments:\n")
 		for _, spec := range positionals {
-			printTwoCols(w, strings.ToUpper(spec.long), spec.help, nil)
+			printTwoCols(w, strings.ToUpper(spec.long), spec.help, "")
 		}
 	}
 
@@ -165,7 +163,7 @@ func (p *Parser) writeHelpForCommand(w io.Writer, cmd *command) {
 	if len(cmd.subcommands) > 0 {
 		fmt.Fprint(w, "\nCommands:\n")
 		for _, subcmd := range cmd.subcommands {
-			printTwoCols(w, subcmd.name, subcmd.help, nil)
+			printTwoCols(w, subcmd.name, subcmd.help, "")
 		}
 	}
 }
@@ -175,29 +173,7 @@ func (p *Parser) printOption(w io.Writer, spec *spec) {
 	if spec.short != "" {
 		left += ", " + synopsis(spec, "-"+spec.short)
 	}
-
-	// If spec.dest is not the zero value then a default value has been added.
-	var v reflect.Value
-	if len(spec.dest.fields) > 0 {
-		v = p.val(spec.dest)
-	}
-
-	var defaultVal *string
-	if v.IsValid() {
-		z := reflect.Zero(v.Type())
-		if (v.Type().Comparable() && z.Type().Comparable() && v.Interface() != z.Interface()) || v.Kind() == reflect.Slice && !v.IsNil() {
-			if scalar, ok := v.Interface().(encoding.TextMarshaler); ok {
-				if value, err := scalar.MarshalText(); err != nil {
-					defaultVal = ptrTo(fmt.Sprintf("error: %v", err))
-				} else {
-					defaultVal = ptrTo(fmt.Sprintf("%v", string(value)))
-				}
-			} else {
-				defaultVal = ptrTo(fmt.Sprintf("%v", v))
-			}
-		}
-	}
-	printTwoCols(w, left, spec.help, defaultVal)
+	printTwoCols(w, left, spec.help, spec.defaultVal)
 }
 
 func synopsis(spec *spec, form string) string {

usage_test.go

@@ -96,26 +96,37 @@ func (n *MyEnum) MarshalText() ([]byte, error) {
 	return nil, errors.New("There was a problem")
 }
 
-func TestUsageError(t *testing.T) {
-	expectedHelp := `Usage: example [--name NAME]
+func TestUsageWithDefaults(t *testing.T) {
+	expectedHelp := `Usage: example [--label LABEL] [--content CONTENT]
 
 Options:
-  --name NAME [default: error: There was a problem]
+  --label LABEL [default: cat]
+  --content CONTENT [default: dog]
   --help, -h             display this help and exit
 `
+	var args struct {
+		Label   string
+		Content string `default:"dog"`
+	}
+	args.Label = "cat"
+	p, err := NewParser(Config{"example"}, &args)
+	require.NoError(t, err)
+
+	args.Label = "should_ignore_this"
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp, help.String())
+}
+
+func TestUsageCannotMarshalToString(t *testing.T) {
 	var args struct {
 		Name *MyEnum
 	}
 	v := MyEnum(42)
 	args.Name = &v
-	p, err := NewParser(Config{"example"}, &args)
-
-	// NB: some might might expect there to be an error here
-	require.NoError(t, err)
-
-	var help bytes.Buffer
-	p.WriteHelp(&help)
-	assert.Equal(t, expectedHelp, help.String())
+	_, err := NewParser(Config{"example"}, &args)
+	assert.EqualError(t, err, `args.Name: error marshaling default value to string: There was a problem`)
 }
 
 func TestUsageLongPositionalWithHelp_legacyForm(t *testing.T) {