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package flaggy
import (
// Parser represents the set of vars and subcommands we are expecting
// from our input args, and the parser than handles them all.
type Parser struct {
Version string // the optional version of the parser.
ShowHelpWithHFlag bool // display help when -h or --help passed
ShowVersionWithVersionFlag bool // display the version when --version passed
ShowHelpOnUnexpected bool // display help when an unexpected flag is passed
TrailingArguments []string // everything after a -- is placed here
HelpTemplate *template.Template // template for Help output
trailingArgumentsExtracted bool // indicates that trailing args have been parsed and should not be appended again
parsed bool // indicates this parser has parsed
subcommandContext *Subcommand // points to the most specific subcommand being used
// NewParser creates a new ArgumentParser ready to parse inputs
func NewParser(name string) *Parser {
// this can not be done inline because of struct embedding
p := &Parser{}
p.Name = name
p.Version = defaultVersion
p.ShowHelpOnUnexpected = true
p.ShowHelpWithHFlag = true
p.ShowVersionWithVersionFlag = true
p.subcommandContext = &Subcommand{}
return p
// ParseArgs parses as if the passed args were the os.Args, but without the
// binary at the 0 position in the array. An error is returned if there
// is a low level issue converting flags to their proper type. No error
// is returned for invalid arguments or missing require subcommands.
func (p *Parser) ParseArgs(args []string) error {
if p.parsed {
return errors.New("Parser.Parse() called twice on parser with name: " + " " + p.Name + " " + p.ShortName)
p.parsed = true
// debugPrint("Kicking off parsing with args:", args)
return p.parse(p, args, 0)
// ShowVersionAndExit shows the version of this parser
func (p *Parser) ShowVersionAndExit() {
fmt.Println("Version:", p.Version)
// SetHelpTemplate sets the go template this parser will use when rendering
// Help.
func (p *Parser) SetHelpTemplate(tmpl string) error {
var err error
p.HelpTemplate = template.New(helpFlagLongName)
p.HelpTemplate, err = p.HelpTemplate.Parse(tmpl)
if err != nil {
return err
return nil
// Parse calculates all flags and subcommands
func (p *Parser) Parse() error {
err := p.ParseArgs(os.Args[1:])
if err != nil {
return err
return nil
// ShowHelp shows Help without an error message
func (p *Parser) ShowHelp() {
debugPrint("showing help for", p.subcommandContext.Name)
// ShowHelpAndExit shows parser help and exits with status code 2
func (p *Parser) ShowHelpAndExit(message string) {
// ShowHelpWithMessage shows the Help for this parser with an optional string error
// message as a header. The supplied subcommand will be the context of Help
// displayed to the user.
func (p *Parser) ShowHelpWithMessage(message string) {
// create a new Help values template and extract values into it
help := Help{}
help.ExtractValues(p, message)
err := p.HelpTemplate.Execute(os.Stderr, help)
if err != nil {
fmt.Fprintln(os.Stderr, "Error rendering Help template:", err)
// Disable show version with --version. It is enabled by default.
func (p *Parser) DisableShowVersionWithVersion() {
p.ShowVersionWithVersionFlag = false
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