Added option for processing environment variables but not setting val…

…ues from them. (#9)

flagset.go

@@ -178,7 +178,7 @@ func (f *FlagSetFiller) processField(flagSet *flag.FlagSet, fieldRef interface{}
 		return err
 	}
 
-	if envName != "" {
+	if !f.options.noSetFromEnv && envName != "" {
 		if val, exists := os.LookupEnv(envName); exists {
 			err := flagSet.Lookup(renamed).Value.Set(val)
 			if err != nil {

flagset_test.go

@@ -772,6 +772,34 @@ func TestWithEnvOverrideDisable(t *testing.T) {
 `, buf.String())
 }
 
+func TestNoSetFromEnv(t *testing.T) {
+	type Config struct {
+		Host string `usage:"arg only"`
+	}
+
+	var config Config
+
+	assert.NoError(t, os.Setenv("APP_HOST", "host from env"))
+
+	filler := flagsfiller.New(
+		flagsfiller.WithEnv("App"),
+		flagsfiller.NoSetFromEnv(),
+	)
+
+	var flagset flag.FlagSet
+	err := filler.Fill(&flagset, &config)
+	require.NoError(t, err)
+
+	buf := grabUsage(flagset)
+
+	assert.Empty(t, config.Host)
+
+	assert.Equal(t, `
+  -host string
+    	arg only (env APP_HOST)
+`, buf.String())
+}
+
 func TestFlagNameOverride(t *testing.T) {
 	type Config struct {
 		Host        string `flag:"server_address" usage:"address of server"`

options.go

@@ -15,6 +15,7 @@ type FillerOption func(opt *fillerOptions)
 type fillerOptions struct {
 	fieldRenamer []Renamer
 	envRenamer   []Renamer
+	noSetFromEnv bool
 }
 
 // WithFieldRenamer declares an option to customize the Renamer used to convert field names
@@ -41,6 +42,15 @@ func WithEnvRenamer(renamer Renamer) FillerOption {
 	}
 }
 
+// NoSetFromEnv ignores setting values from the environment.
+// All environment variable renamers are run but values are not set from the environment.
+// This is good to use if you need to build a flag set with default values that don't consider the current environment.
+func NoSetFromEnv() FillerOption {
+	return func(opt *fillerOptions) {
+		opt.noSetFromEnv = true
+	}
+}
+
 func (o *fillerOptions) renameLongName(name string) string {
 	if len(o.fieldRenamer) == 0 {
 		return DefaultFieldRenamer(name)