cli: use FlagSet output instead of mutating it

cmd.go

@@ -20,6 +20,7 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"os"
 	"strings"
 )
 
@@ -52,19 +53,22 @@ type Command struct {
 }
 
 // Help writes the usage line, flags, and description for the command to the
-// provided io.Writer.
-// If c.Flags is a valid flag set, calling Help sets the output of c.Flags.
-func (c *Command) Help(w io.Writer) {
+// flag set's output or to stdout if Flags is nil.
+func (c *Command) Help() {
 	if c == nil {
 		return
 	}
+
+	var w io.Writer = os.Stdout
+	if c.Flags != nil {
+		w = c.Flags.Output()
+	}
 	// If there is a usage line and it's more than just the name, print it.
 	if c.Usage != "" && c.Name() != c.Usage {
 		fmt.Fprintf(w, "Usage: %s\n\n", c.Usage)
 	}
 	if c.Flags != nil {
 		fmt.Fprint(w, "Options:\n\n")
-		c.Flags.SetOutput(w)
 		c.Flags.PrintDefaults()
 		fmt.Fprintln(w, "")
 	}

cmd_test.go

@@ -39,11 +39,12 @@ func TestCommand(t *testing.T) {
 			}
 		})
 		t.Run(fmt.Sprintf("Help/%d", i), func(t *testing.T) {
-			tc.cmd.Flags = flag.NewFlagSet("testflags", flag.ExitOnError)
+			tc.cmd.Flags = flag.NewFlagSet("testflags", flag.PanicOnError)
 			tc.cmd.Flags.String("testflag", "testflagvalue", "usage of a test flag")
 
 			b.Reset()
-			tc.cmd.Help(b)
+			tc.cmd.Flags.SetOutput(b)
+			tc.cmd.Help()
 			if tc.cmd.Run != nil && !bytes.Contains(b.Bytes(), []byte(tc.cmd.Usage)) {
 				t.Errorf("Expected cmd.Help() output to contain cmd.Usage")
 			}
@@ -125,7 +126,11 @@ func TestCommandSet(t *testing.T) {
 		if tc.cs != nil {
 			t.Run(fmt.Sprintf("Help/%d", i), func(t *testing.T) {
 				b := new(bytes.Buffer)
-				tc.cs.Help(b)
+				if tc.cs.Flags == nil {
+					tc.cs.Flags = flag.NewFlagSet("t", flag.PanicOnError)
+				}
+				tc.cs.Flags.SetOutput(b)
+				tc.cs.Help()
 				for _, cmd := range tc.cs.Commands {
 					if !bytes.Contains(b.Bytes(), []byte(cmd.Name())) {
 						t.Errorf("Expected commandset help to contain command name: %s", cmd.Name())

example_test.go

@@ -14,6 +14,7 @@ import (
 
 func commitCmd(cfg *string) *cli.Command {
 	commitFlags := flag.NewFlagSet("commit", flag.ExitOnError)
+	commitFlags.SetOutput(os.Stdout)
 	help := commitFlags.Bool("h", false, "Print this commands help output…")
 	interactive := commitFlags.Bool("interactive", false, "Run commit in interactive mode.")
 	if cfg == nil {
@@ -35,7 +36,7 @@ Stores the current contents of the index in a new commit…`,
 				fmt.Println("Interactive mode enabled.")
 			}
 			if *help {
-				c.Help(os.Stdout)
+				c.Help()
 			}
 			return nil
 		},

help.go

@@ -6,11 +6,11 @@ package cli
 
 import (
 	"fmt"
-	"os"
 )
 
 // Help returns a Command that prints help information about its command set to
-// stdout, or about a specific command if one is provided as an argument.
+// the command's Help output, or information about a specific command if one is
+// provided as an argument.
 //
 // For example, in a program called "git" running:
 //
@@ -24,7 +24,7 @@ func Help(cs *Command) *Command {
 		Run: func(c *Command, args ...string) error {
 			// If there aren't any arguments, print the main command help.
 			if len(args) == 0 {
-				cs.Help(os.Stdout)
+				cs.Help()
 				return nil
 			}
 
@@ -35,7 +35,7 @@ func Help(cs *Command) *Command {
 				}
 				// If this is the article, run its help command.
 				if len(args) == 1 {
-					cmd.Help(os.Stdout)
+					cmd.Help()
 					return nil
 				}