golang - Using and testing `flag`

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package main  
  
import (  
   "flag"  
   "fmt"   "testing")  
  
var flagDebug bool  
var flagHost  string  
  
func init() {  
   flag.BoolVar(&flagDebug, "debug", false, "enable debug output")  
   flag.StringVar(&flagHost, "host", "example.com", "set the host to connect to")  
   flag.Parse()  
}  
  
func TestFlags(t *testing.T) {  
   fmt.Println(flagDebug)  
   fmt.Println(flagHost)    
}  
  
func TestFlagsAgain(t *testing.T) {  
   fmt.Println(flagDebug)  
   fmt.Println(flagHost)    
}

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flag provided but not defined: -test.v
Usage of /private/var/folders/bk/k_kbqfsx0qgcdj7t4jn9g5dc0000gn/T/GoLand/___TestFlags_in_flags_example_test_go.test:
  -debug
    	enable debug output
  -host string
    	set the host to connect to (default "example.com")

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package main  
  
import (  
   "flag"  
   "fmt"   "os"   "testing")  
  
var flagDebug bool  
var flagHost  string  
  
func parseFlags() {  
   flag.BoolVar(&flagDebug, "debug", false, "enable debug output")  
   flag.StringVar(&flagHost, "host", "example.com", "set the host to connect to")  
   flag.Parse()  
}  
  
func TestAll(t *testing.T) {  
   t.Run("test flags", BasicFlags)  
   t.Run("change args", ChangeArgs)  
}  
  
func BasicFlags(t *testing.T) {  
   parseFlags()  
   fmt.Println(flagDebug)  
   fmt.Println(flagHost)    
}  
  
func ChangeArgs(t *testing.T) {  
   os.Args = []string{"test", "--debug", "false", "--host", "google.com" }  
   parseFlags()  
   fmt.Println(flagDebug)  
   fmt.Println(flagHost)    
}

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=== RUN   TestAll
=== RUN   TestAll/test_flags
false
example.com
=== RUN   TestAll/change_args
/private/var/folders/bk/k_kbqfsx0qgcdj7t4jn9g5dc0000gn/T/GoLand/___TestAll_in_flags_example_test_go.test flag redefined: debug
--- FAIL: TestAll (0.00s)
    --- PASS: TestAll/test_flags (0.00s)
    --- FAIL: TestAll/change_args (0.00s)
panic: /private/var/folders/bk/k_kbqfsx0qgcdj7t4jn9g5dc0000gn/T/GoLand/___TestAll_in_flags_example_test_go.test flag redefined: debug [recovered]
	panic: /private/var/folders/bk/k_kbqfsx0qgcdj7t4jn9g5dc0000gn/T/GoLand/___TestAll_in_flags_example_test_go.test flag redefined: debug

goroutine 6 [running]:
testing.tRunner.func1.2({0x102f28240, 0x1400005a5d0})
	/usr/local/go/src/testing/testing.go:1209 +0x258
testing.tRunner.func1(0x140001189c0)
	/usr/local/go/src/testing/testing.go:1212 +0x284
panic({0x102f28240, 0x1400005a5d0})
	/usr/local/go/src/runtime/panic.go:1038 +0x21c
flag.(*FlagSet).Var(0x14000070120, {0x102f48100, 0x103027c80}, {0x102ef27b6, 0x5}, {0x102ef5485, 0x13})
	/usr/local/go/src/flag/flag.go:879 +0x2d8
flag.BoolVar(...)
	/usr/local/go/src/flag/flag.go:638
command-line-arguments.parseFlags()
	/Users/shermanrose/Library/Application Support/JetBrains/GoLand2022.1/scratches/flags_example_test.go:13 +0x74
command-line-arguments.ChangeArgs(0x140001189c0)
	/Users/shermanrose/Library/Application Support/JetBrains/GoLand2022.1/scratches/flags_example_test.go:31 +0xdc
testing.tRunner(0x140001189c0, 0x102f45550)
	/usr/local/go/src/testing/testing.go:1259 +0xfc
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1306 +0x328

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package main  
  
import (  
   "flag"  
   "testing")  
  
func TestAll(t *testing.T) {  
     
   tests  := map[string]struct{  
      args []string  
      wantsDebug bool  
      wantsHost string  
   }{  
      "args set 1" : {  
         args: []string{"prog", "--debug", "--host", "google.com"},    
         wantsDebug: true,  
         wantsHost: "google.com",  
       },  
      "args set 2" : {  
         args: []string{"prog", "--host", "github.com"},  
         wantsDebug: false,  
         wantsHost: "github.com",  
      },  
   }  
     
   for name, data :=  range tests {  
      t.Run(name, func(t *testing.T) {  
         var flagDebug bool  
         var flagHost string  
      flags := flag.NewFlagSet(data.args[0], flag.ContinueOnError)  
         flags.BoolVar(&flagDebug, "debug", false, "enable debug logging")  
         flags.StringVar(&flagHost, "host", "example.com", "set the host to connect to")  
           
         err := flags.Parse(data.args[1:])  
         if err != nil {  
            t.Fatalf("failed to parse args: %s", err.Error())    
         }  
           
         if flagDebug != data.wantsDebug {  
            t.Fatalf("didn't get expected debug value")  
         }  
  
         if flagHost != data.wantsHost{  
            t.Fatalf("didn't get expected host value")  
         }  
        
      })  
   }  
     
}