Add 'IgnoreDefault' option

README.md

@@ -180,6 +180,24 @@ var args struct {
 arg.MustParse(&args)
 ```
 
+#### Ignoring environment variables and/or default values
+
+The values in an existing structure can be kept in-tact by ignoring environment
+variables and/or default values.
+
+```go
+var args struct {
+    Test  string `arg:"-t,env:TEST" default:"something"`
+}
+
+p, err := arg.NewParser(arg.Config{
+    IgnoreEnv: true,
+    IgnoreDefault: true,
+}, &args)
+
+err = p.Parse(os.Args)
+```
+
 ### Arguments with multiple values
 ```go
 var args struct {

parse.go

@@ -121,6 +121,10 @@ type Config struct {
 
 	// IgnoreEnv instructs the library not to read environment variables
 	IgnoreEnv bool
+
+	// IgnoreDefault instructs the library not to reset the variables to the
+	// default values, including pointers to sub commands
+	IgnoreDefault bool
 }
 
 // Parser represents a set of command line options with destination values
@@ -527,7 +531,9 @@ func (p *Parser) process(args []string) error {
 
 			// instantiate the field to point to a new struct
 			v := p.val(subcmd.dest)
-			v.Set(reflect.New(v.Type().Elem())) // we already checked that all subcommands are struct pointers
+			if !p.config.IgnoreDefault || v.IsNil() {
+				v.Set(reflect.New(v.Type().Elem())) // we already checked that all subcommands are struct pointers
+			}
 
 			// add the new options to the set of allowed options
 			specs = append(specs, subcmd.specs...)
@@ -659,7 +665,7 @@ func (p *Parser) process(args []string) error {
 			}
 			return errors.New(msg)
 		}
-		if spec.defaultVal != "" {
+		if !p.config.IgnoreDefault && spec.defaultVal != "" {
 			err := scalar.ParseValue(p.val(spec.dest), spec.defaultVal)
 			if err != nil {
 				return fmt.Errorf("error processing default value for %s: %v", name, err)

parse_test.go

@@ -816,6 +816,19 @@ func TestEnvironmentVariableIgnored(t *testing.T) {
 	assert.Equal(t, "", args.Foo)
 }
 
+func TestDefaultValuesIgnored(t *testing.T) {
+	var args struct {
+		Foo string `default:"bad"`
+	}
+
+	p, err := NewParser(Config{IgnoreDefault: true}, &args)
+	require.NoError(t, err)
+
+	err = p.Parse(nil)
+	assert.NoError(t, err)
+	assert.Equal(t, "", args.Foo)
+}
+
 func TestEnvironmentVariableInSubcommandIgnored(t *testing.T) {
 	var args struct {
 		Sub *struct {