1 files changed, 36 insertions, 29 deletions

diff --git a/parse.go b/parse.go
index 6d8b509..be77924 100644
--- a/parse.go
+++ b/parse.go
@@ -75,13 +75,28 @@ var ErrHelp = errors.New("help requested by user")
 // ErrVersion indicates that --version was provided
 var ErrVersion = errors.New("version requested by user")
 
+// for monkey patching in example code
+var mustParseExit = os.Exit
+
 // MustParse processes command line arguments and exits upon failure
 func MustParse(dest ...interface{}) *Parser {
-	p, err := NewParser(Config{}, dest...)
+	return mustParse(Config{Exit: mustParseExit}, dest...)
+}
+
+// mustParse is a helper that facilitates testing
+func mustParse(config Config, dest ...interface{}) *Parser {
+	if config.Exit == nil {
+		config.Exit = os.Exit
+	}
+	if config.Out == nil {
+		config.Out = os.Stdout
+	}
+
+	p, err := NewParser(config, dest...)
 	if err != nil {
-		fmt.Fprintln(stdout, err)
-		osExit(-1)
-		return nil // just in case osExit was monkey-patched
+		fmt.Fprintln(config.Out, err)
+		config.Exit(-1)
+		return nil
 	}
 
 	p.MustParse(flags())
@@ -121,9 +136,11 @@ type Config struct {
 	// subcommand
 	StrictSubcommands bool
 
-	OsExit func(int)
-	Stdout io.Writer
-	Stderr io.Writer
+	// Exit is called to terminate the process with an error code (defaults to os.Exit)
+	Exit func(int)
+
+	// Out is where help text, usage text, and failure messages are printed (defaults to os.Stdout)
+	Out io.Writer
 }
 
 // Parser represents a set of command line options with destination values
@@ -137,10 +154,6 @@ type Parser struct {
 
 	// the following field changes during processing of command line arguments
 	lastCmd *command
-
-	osExit func(int)
-	stdout io.Writer
-	stderr io.Writer
 }
 
 // Versioned is the interface that the destination struct should implement to
@@ -190,6 +203,14 @@ func walkFieldsImpl(t reflect.Type, visit func(field reflect.StructField, owner
 
 // NewParser constructs a parser from a list of destination structs
 func NewParser(config Config, dests ...interface{}) (*Parser, error) {
+	// fill in defaults
+	if config.Exit == nil {
+		config.Exit = os.Exit
+	}
+	if config.Out == nil {
+		config.Out = os.Stdout
+	}
+
 	// first pick a name for the command for use in the usage text
 	var name string
 	switch {
@@ -205,20 +226,6 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 	p := Parser{
 		cmd:    &command{name: name},
 		config: config,
-
-		osExit: osExit,
-		stdout: stdout,
-		stderr: stderr,
-	}
-
-	if config.OsExit != nil {
-		p.osExit = config.OsExit
-	}
-	if config.Stdout != nil {
-		p.stdout = config.Stdout
-	}
-	if config.Stderr != nil {
-		p.stderr = config.Stderr
 	}
 
 	// make a list of roots
@@ -506,11 +513,11 @@ func (p *Parser) MustParse(args []string) {
 	err := p.Parse(args)
 	switch {
 	case err == ErrHelp:
-		p.writeHelpForSubcommand(p.stdout, p.lastCmd)
-		p.osExit(0)
+		p.writeHelpForSubcommand(p.config.Out, p.lastCmd)
+		p.config.Exit(0)
 	case err == ErrVersion:
-		fmt.Fprintln(p.stdout, p.version)
-		p.osExit(0)
+		fmt.Fprintln(p.config.Out, p.version)
+		p.config.Exit(0)
 	case err != nil:
 		p.failWithSubcommand(err.Error(), p.lastCmd)
 	}