Add capability to restrict flag values to a set of allowed values

[ian-howell]

I have a use-case where I need to perform validation on flag values. In other words, I need to check that the value passed in by the user belongs to some set of valid values. I end up writing a lot of code that looks like this:

package main

import (
	"fmt"
	"os"

	flag "github.com/spf13/pflag"
)

var color string

func main() {
	addFlags()
	flag.Parse()
	if err := validateFlags(); err != nil {
		fmt.Fprintf(os.Stderr, "Error: %v", err)
		os.Exit(1)
	}
	fmt.Printf("Selected color is %s\n", color)
}

func addFlags() {
	flag.StringVar(&color, "color", "", "The color for the example")
}

func validateFlags() error {
	validColors := []string{"red", "blue", "yellow"}
	for _, validColor := range validColors {
		if color == validColor {
			// color is valid
			return nil
		}
	}
	// if we're here, color is invalid
	return fmt.Errorf("Value '%s' is invalid for flag 'color'. Valid values "+
		"come from the set %v", color, validColors)
}

I'd like to be able to do something like the following:

	flag.StringVarRestricted(
		&color,                            // pointer to variable
		"color",                           // flag name
		"",                                // default value
		"The color for the example",       // description
		[]string{"red", "blue", "yellow"}) // Allowed values

I wouldn't mind taking a shot at implementing something like this if it sounds acceptable.

[macintacos]

Just dropping a comment noting that this is the most thumbs-upped issue in this repo, and I'd love to see it come to fruition 🙂

Right now I do some reflection-gymnastics based on pre-defined types to validate and define flags, so if this were added in this project it'd obviate the need for that custom logic and make my codebase that much easier to maintain. I see #237 was opened, but looks like it didn't get much traction?

[d3an]

Python's argparse supports flags with predefined choices. I want to replicate the same behaviour in Cobra, and think it would be a great addition.

[migueleliasweb]

This is would be so useful. Is there anything keeping this from getting merged? 🙏

[felipe-avelar]

I think this feature would be awesome, I would like to suggest a change, instead a slice of valid values could be a bool function that validates the value 😃

[pmenglund]

@felipe-avelar that is a better approach as it is more flexiable, so using the initial example it would become:

	flag.StringVarValidated(
		&color,                            // pointer to variable
		"color",                           // flag name
		"",                                // default value
		"The color for the example",       // description
		func (s string) bool { ... }(),    // Function to evaluate valid values

[fastcat]

Alternative POV: If you provide a slice of valid values, then help output can tell the user what they are, and shell autocomplete integration can leverage that information.

Of course, not all cases can be handled by a slice, so this is probably a case of "yes, and". For example, could have the validation function, and an (optional) slice of "suggested" values?