feature: accept custom Environment map

parse.go

@@ -141,6 +141,9 @@ type Config struct {
 
 	// Out is where help text, usage text, and failure messages are printed (defaults to os.Stdout)
 	Out io.Writer
+
+	// Environment is a map of environment variables to override those in the process environment, or provide values to those not in the process environment.
+	Environment map[string]string
 }
 
 // Parser represents a set of command line options with destination values
@@ -531,7 +534,17 @@ func (p *Parser) captureEnvVars(specs []*spec, wasPresent map[*spec]bool) error
 			continue
 		}
 
-		value, found := os.LookupEnv(spec.env)
+		var value string
+		var found bool
+
+		if !p.config.IgnoreEnv {
+			value, found = os.LookupEnv(spec.env)
+		}
+
+		if p.config.Environment != nil {
+			value, found = p.config.Environment[spec.env]
+		}
+
 		if !found {
 			continue
 		}
@@ -584,7 +597,7 @@ func (p *Parser) process(args []string) error {
 	copy(specs, curCmd.specs)
 
 	// deal with environment vars
-	if !p.config.IgnoreEnv {
+	if !p.config.IgnoreEnv || p.config.Environment != nil {
 		err := p.captureEnvVars(specs, wasPresent)
 		if err != nil {
 			return err
@@ -640,7 +653,7 @@ func (p *Parser) process(args []string) error {
 			}
 
 			// capture environment vars for these new options
-			if !p.config.IgnoreEnv {
+			if !p.config.IgnoreEnv || p.config.Environment != nil {
 				err := p.captureEnvVars(subcmd.specs, wasPresent)
 				if err != nil {
 					return err

parse_test.go

@@ -39,7 +39,13 @@ func parseWithEnv(tb testing.TB, cmdline string, env []string, dest interface{})
 }
 
 func parseWithEnvErr(tb testing.TB, cmdline string, env []string, dest interface{}) (*Parser, error) {
-	p, err := NewParser(Config{}, dest)
+	tb.Helper()
+	return parseWithConfigEnvErr(tb, Config{}, cmdline, env, dest)
+}
+
+func parseWithConfigEnvErr(tb testing.TB, config Config, cmdline string, env []string, dest interface{}) (*Parser, error) {
+	tb.Helper()
+	p, err := NewParser(config, dest)
 	if err != nil {
 		return nil, err
 	}
@@ -669,6 +675,24 @@ func TestEnvironmentVariable(t *testing.T) {
 	assert.Equal(t, "bar", args.Foo)
 }
 
+func TestEnvironmentVariableViaCustomEnvironment(t *testing.T) {
+	var args struct {
+		Foo string `arg:"env"`
+	}
+	_, err := parseWithConfigEnvErr(t, Config{Environment: map[string]string{"FOO": "bar"}}, "", nil, &args)
+	require.NoError(t, err)
+	assert.Equal(t, "bar", args.Foo)
+}
+
+func TestEnvironmentVariableOverriddenByCustomEnvironment(t *testing.T) {
+	var args struct {
+		Foo string `arg:"env"`
+	}
+	_, err := parseWithConfigEnvErr(t, Config{Environment: map[string]string{"FOO": "bar"}}, "", []string{"FOO=foo"}, &args)
+	require.NoError(t, err)
+	assert.Equal(t, "bar", args.Foo)
+}
+
 func TestEnvironmentVariableNotPresent(t *testing.T) {
 	var args struct {
 		NotPresent string `arg:"env"`