Making StringMap/StringMapVar optional?

[SlowPokeInTexas]

Greetings, this is my favorite of the Go command-line parsing mechanisms. One question though: I noticed that StringMap/StringMapVar have a minimum of 1 entry. In my case I intend to fall back to a configuration file for map[string]string values that aren't explicitly provided on the command-line. Does this sound reasonable or am I making an incorrect usage assumption?

[DavidGamba]

Hi @SlowPokeInTexas it is awesome to know that someone else besides me and my team are using the tool.

I noticed that StringMap/StringMapVar have a minimum of 1 entry. In my case I intend to fall back to a configuration file for map[string]string values that aren't explicitly provided on the command-line.

I normally accomplish what I believe you are trying to say in the following way:

Given this program main.txt:

package main

import (
	"fmt"
	"io/ioutil"
	"log"
	"os"

	"github.com/DavidGamba/go-getoptions"
)

var logger = log.New(ioutil.Discard, "", log.LstdFlags)

func main() {
	config := map[string]string{
		"region":  "us-east-1",
		"profile": "default",
	}
	opt := getoptions.New()
	opt.Bool("help", false, opt.Alias("?"))
	opt.Bool("debug", false)
	opt.StringMapVar(&config, "config", 1, 99)
	remaining, err := opt.Parse(os.Args[1:])
	if err != nil {
		fmt.Fprintf(os.Stderr, "ERROR: %s\n", err)
		os.Exit(1)
	}
	if opt.Called("help") {
		fmt.Fprintln(os.Stderr, opt.Help())
		os.Exit(1)
	}
	if opt.Called("debug") {
		logger.SetOutput(os.Stderr)
	}
	log.Println(remaining)

	fmt.Printf("%#v\n", config)
}

You can run the following on the cli:

$ go run main.go                                                                                                                                                                                                                                          
2019/09/12 08:42:22 []
map[string]string{"profile":"default", "region":"us-east-1"}

$ go run main.go -config profile=hello    
2019/09/12 08:42:32 []                                                                                                                                                                                                                                    
map[string]string{"profile":"hello", "region":"us-east-1"}   

$ go run main.go -config profile=hello region=us-west-2                                                                                                                                                                                                   
2019/09/12 08:42:48 []
map[string]string{"profile":"hello", "region":"us-west-2"}

So you can see that if you always pre-load the config map, its entries will not be overwritten unless explicitly passed.

Another option to do the same, is to have two maps, one for the config unmarshaling and one for the config overrides, then you merge them by iterating over the second and overriding the first one.

Something like main2.txt:

package main

import (
	"fmt"
	"io/ioutil"
	"log"
	"os"

	"github.com/DavidGamba/go-getoptions"
)

var logger = log.New(ioutil.Discard, "", log.LstdFlags)

func main() {
	var configFile string
	var configOverride map[string]string

	opt := getoptions.New()
	opt.Bool("help", false, opt.Alias("?"))
	opt.Bool("debug", false)
	opt.StringVar(&configFile, "file", "config.yaml")
	opt.StringMapVar(&configOverride, "config", 1, 99)
	remaining, err := opt.Parse(os.Args[1:])
	if err != nil {
		fmt.Fprintf(os.Stderr, "ERROR: %s\n", err)
		os.Exit(1)
	}
	if opt.Called("help") {
		fmt.Fprintln(os.Stderr, opt.Help())
		os.Exit(1)
	}
	if opt.Called("debug") {
		logger.SetOutput(os.Stderr)
	}
	log.Println(remaining)

	// TODO: Unmarshall configFile into config var
	// ...
	config := map[string]string{
		"region":  "us-east-1",
		"profile": "default",
	}

	// Override defaults with configOverride
	for k, v := range configOverride {
		config[k] = v
	}

	fmt.Printf("%#v\n", config)
}

With the same results:

$ go run main.go
2019/09/12 08:55:29 []
map[string]string{"profile":"default", "region":"us-east-1"}

$ go run main.go -config profile=hello
2019/09/12 08:55:42 []
map[string]string{"profile":"hello", "region":"us-east-1"}

$ go run main.go -config profile=hello region=us-west-2
2019/09/12 08:55:52 []
map[string]string{"profile":"hello", "region":"us-west-2"}

Let me you what you think of this approach.

[SlowPokeInTexas]

Thank you David. Just now getting back to this project; what you suggest works fine.

Best Regards!