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fix: --long= should not consume the next argument #139

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fix: --long= should not consume the next argument #139

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28 changes: 11 additions & 17 deletions flag_set.go
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Original file line number Diff line number Diff line change
Expand Up @@ -241,16 +241,8 @@ func (fs *FlagSet) parseShortFlag(arg string, args []string) ([]string, error) {
}

func (fs *FlagSet) parseLongFlag(arg string, args []string) ([]string, error) {
var (
name string
value string
)

if equals := strings.IndexRune(arg, '='); equals > 0 {
arg, value = arg[:equals], arg[equals+1:]
}

name = strings.TrimPrefix(arg, "--")
name, value, eqFound := strings.Cut(arg, "=")
name = strings.TrimPrefix(name, "--")

f := fs.findLongFlag(name)
if f == nil {
Expand All @@ -264,22 +256,24 @@ func (fs *FlagSet) parseLongFlag(arg string, args []string) ([]string, error) {
}
}

if value == "" {
if eqFound && f.isBoolFlag && value == "" {
value = "true" // `--debug=` amounts to `--debug=true`
}

@cbandy cbandy Aug 30, 2025

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I would move this into the switch, if possible.

	switch {
	case hasEquals && f.isBoolFlag && value == "":
		// Treat --bool= the same as --bool=true
		value = "true"

	case hasEquals:
		// Use the value within this arg as-is.

	case f.isBoolFlag:
		// Use the value of the next arg, if it exists and looks related.
		...

	default:
		// Use the value of the next arg, if it exists.
		...
	}

@telemachus telemachus Aug 30, 2025

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I'm happy to look at that, but I'm not going to push any further changes until we hear from @peterbourgon. I'd like to know what he's thinking about this PR before changing it further.


if value == "" && !eqFound {
switch {
case f.isBoolFlag:
value = "true" // `-b` or `--foo` default to true
value = "true" // `--foo` defaults to true
if len(args) > 0 {
if _, err := strconv.ParseBool(args[0]); err == nil {
value = args[0] // `-b true` or `--foo false` should also work
value = args[0] // `--foo false` should also work
args = args[1:]
}
}
Comment on lines 267 to 272

@ccoVeille ccoVeille Jan 17, 2025

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There is also something strange with the code (the code that already exists)

Let's assume --foo expects a string

--foo leads to an error
--foo true leads to "true"
--foo=1 leads to "1"
--foo=false leads to "false"
--foo= leads to ""

But if --foo is a boolean

--foo false
--foo whatever
--foo 0
--foo 2
--foo=2

Leads to this

--foo=false
--foo whatever
--foo=false
--foo=true 2
an error

This behavior is strange to me, but I'm unsure how other libraries parsing flags do

@telemachus telemachus Jan 17, 2025

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The difference between --foo 2 and --foo=2 is...not great. That is probably why flag in Go's standard library restricts the form --flag arg to non-boolean flags.

@ccoVeille ccoVeille Jan 17, 2025

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Exactly

@telemachus telemachus Jan 17, 2025

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I would be in favor of that here as well, but I think @peterbourgon wants to keep boolean parsing as is. (Also, that would probably be a very breaking API change now. Don't know how that plays with v4 being in beta.)

@telemachus telemachus Jan 21, 2025

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@ccoVeille As a side note, flag in the standard library has a parallel inconsistency but for string flags.

	fs := flag.NewFlagSet("whatever", flag.ContinueOnError)
	cfg := struct {
		config  string
		// ...
	}{}
	fs.StringVar(&cfg.config, "config", "", "Use this configuration file")
	// ...
whatever -config=foo   # No error; config is set to "foo"
whatever -config foo   # No error; config is set to "foo"
whatever -config ""    # No error; config is set to ""
whatever -config=""    # No error; config is set to ""
whatever -config       # Error, namely "flag needs an argument: -config"
whatever -config=      # No error; config is set to ""

I think that the last two should return the same error, but they do not. I'm guessing this is because the parser would need to do extra work (not much but some) to detect the difference between -config= and -config="". I doubt anything can be done about it now (breaking API change), but I think it was a mistake.

@ccoVeille ccoVeille Jan 28, 2025

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For me, the behavior of the stdlib is fine here. I might look a bit odd, but I'm fine with it.

It has its logic.

About ff lib, I don't know really. You are fixing a bug with the boolean flag after all, so fixing the bug is somehow already breaking something that was broken 😄, by fixing it.

While your fix is fine, I think the issue of removing the random behavior of boolean flag with a parameter should be considered, at least to have a library that behave like other libraries.

I don't think it was intended, I would remove it. But, except that your PR is fine, you are fixing the behavior with --foo= so nothing about --foo 0

So for me it can be merged as is

@ccoVeille ccoVeille Mar 14, 2025

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For record, I randomly found a discussion about the exact same issue with boolean flag

case !f.isBoolFlag && len(args) > 0:
case len(args) > 0:
value, args = args[0], args[1:]
case !f.isBoolFlag && len(args) <= 0:
return nil, fmt.Errorf("missing value")
default:
panic("unreachable")
return nil, fmt.Errorf("missing value")
}
}

Expand Down
1 change: 1 addition & 0 deletions flag_set_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -83,6 +83,7 @@ func TestFlagSet_Bool(t *testing.T) {
{args: []string{"--help"}, wantX: false, wantY: true, wantErr: ff.ErrHelp},
{args: []string{"--xflag", "-h"}, wantX: true, wantY: true, wantErr: ff.ErrHelp},
{args: []string{"-y", "--help"}, wantX: false, wantY: false, wantErr: ff.ErrHelp},
{args: []string{"--xflag=", "--help"}, wantX: true, wantY: true, wantErr: ff.ErrHelp},
} {
t.Run(strings.Join(test.args, " "), func(t *testing.T) {
fs := ff.NewFlagSet(t.Name())
Expand Down
6 changes: 6 additions & 0 deletions parse_test.go
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Expand Up @@ -233,6 +233,12 @@ func TestParse_FlagSet(t *testing.T) {
Args: []string{`--str`, `foo`, `--help`, `-b`},
Want: fftest.Vars{S: "foo", B: false, WantParseErrorIs: ff.ErrHelp},
},
{
Name: "--str= -a",
Constructors: []fftest.Constructor{fftest.CoreConstructor},
Args: []string{`--str=`, `-a`},
Want: fftest.Vars{S: "", A: true},
},
{
Name: "-s foo -f 1.23",
Constructors: []fftest.Constructor{fftest.CoreConstructor},
Expand Down