Allow customization of prefixes and environment variable hints in flag help strings

flag.go

@@ -37,6 +37,14 @@ var HelpFlag Flag = BoolFlag{
 // to display a flag.
 var FlagStringer FlagStringFunc = stringifyFlag
 
+// FlagNamePrefixer converts a full flag name and its placeholder into the help
+// message flag prefix. This is used by the default FlagStringer.
+var FlagNamePrefixer FlagNamePrefixFunc = prefixedNames
+
+// FlagEnvHinter annotates flag help message with the environment variable
+// details. This is used by the default FlagStringer.
+var FlagEnvHinter FlagEnvHintFunc = withEnvHint
+
 // FlagsByName is a slice of Flag.
 type FlagsByName []Flag
 
@@ -710,13 +718,13 @@ func stringifyFlag(f Flag) string {
 
 	switch f.(type) {
 	case IntSliceFlag:
-		return withEnvHint(fv.FieldByName("EnvVar").String(),
+		return FlagEnvHinter(fv.FieldByName("EnvVar").String(),
 			stringifyIntSliceFlag(f.(IntSliceFlag)))
 	case Int64SliceFlag:
-		return withEnvHint(fv.FieldByName("EnvVar").String(),
+		return FlagEnvHinter(fv.FieldByName("EnvVar").String(),
 			stringifyInt64SliceFlag(f.(Int64SliceFlag)))
 	case StringSliceFlag:
-		return withEnvHint(fv.FieldByName("EnvVar").String(),
+		return FlagEnvHinter(fv.FieldByName("EnvVar").String(),
 			stringifyStringSliceFlag(f.(StringSliceFlag)))
 	}
 
@@ -744,8 +752,8 @@ func stringifyFlag(f Flag) string {
 
 	usageWithDefault := strings.TrimSpace(fmt.Sprintf("%s%s", usage, defaultValueString))
 
-	return withEnvHint(fv.FieldByName("EnvVar").String(),
-		fmt.Sprintf("%s\t%s", prefixedNames(fv.FieldByName("Name").String(), placeholder), usageWithDefault))
+	return FlagEnvHinter(fv.FieldByName("EnvVar").String(),
+		fmt.Sprintf("%s\t%s", FlagNamePrefixer(fv.FieldByName("Name").String(), placeholder), usageWithDefault))
 }
 
 func stringifyIntSliceFlag(f IntSliceFlag) string {
@@ -795,5 +803,5 @@ func stringifySliceFlag(usage, name string, defaultVals []string) string {
 	}
 
 	usageWithDefault := strings.TrimSpace(fmt.Sprintf("%s%s", usage, defaultVal))
-	return fmt.Sprintf("%s\t%s", prefixedNames(name, placeholder), usageWithDefault)
+	return fmt.Sprintf("%s\t%s", FlagNamePrefixer(name, placeholder), usageWithDefault)
 }

flag_test.go

@@ -158,6 +158,83 @@ func TestStringFlagWithEnvVarHelpOutput(t *testing.T) {
 	}
 }
 
+var prefixStringFlagTests = []struct {
+	name     string
+	usage    string
+	value    string
+	prefixer FlagNamePrefixFunc
+	expected string
+}{
+	{"foo", "", "", func(a, b string) string {
+		return fmt.Sprintf("name: %s, ph: %s", a, b)
+	}, "name: foo, ph: value\t"},
+	{"f", "", "", func(a, b string) string {
+		return fmt.Sprintf("name: %s, ph: %s", a, b)
+	}, "name: f, ph: value\t"},
+	{"f", "The total `foo` desired", "all", func(a, b string) string {
+		return fmt.Sprintf("name: %s, ph: %s", a, b)
+	}, "name: f, ph: foo\tThe total foo desired (default: \"all\")"},
+	{"test", "", "Something", func(a, b string) string {
+		return fmt.Sprintf("name: %s, ph: %s", a, b)
+	}, "name: test, ph: value\t(default: \"Something\")"},
+	{"config,c", "Load configuration from `FILE`", "", func(a, b string) string {
+		return fmt.Sprintf("name: %s, ph: %s", a, b)
+	}, "name: config,c, ph: FILE\tLoad configuration from FILE"},
+	{"config,c", "Load configuration from `CONFIG`", "config.json", func(a, b string) string {
+		return fmt.Sprintf("name: %s, ph: %s", a, b)
+	}, "name: config,c, ph: CONFIG\tLoad configuration from CONFIG (default: \"config.json\")"},
+}
+
+func TestFlagNamePrefixer(t *testing.T) {
+	defer func() {
+		FlagNamePrefixer = prefixedNames
+	}()
+
+	for _, test := range prefixStringFlagTests {
+		FlagNamePrefixer = test.prefixer
+		flag := StringFlag{Name: test.name, Usage: test.usage, Value: test.value}
+		output := flag.String()
+		if output != test.expected {
+			t.Errorf("%q does not match %q", output, test.expected)
+		}
+	}
+}
+
+var envHintFlagTests = []struct {
+	name     string
+	env      string
+	hinter   FlagEnvHintFunc
+	expected string
+}{
+	{"foo", "", func(a, b string) string {
+		return fmt.Sprintf("env: %s, str: %s", a, b)
+	}, "env: , str: --foo value\t"},
+	{"f", "", func(a, b string) string {
+		return fmt.Sprintf("env: %s, str: %s", a, b)
+	}, "env: , str: -f value\t"},
+	{"foo", "ENV_VAR", func(a, b string) string {
+		return fmt.Sprintf("env: %s, str: %s", a, b)
+	}, "env: ENV_VAR, str: --foo value\t"},
+	{"f", "ENV_VAR", func(a, b string) string {
+		return fmt.Sprintf("env: %s, str: %s", a, b)
+	}, "env: ENV_VAR, str: -f value\t"},
+}
+
+func TestFlagEnvHinter(t *testing.T) {
+	defer func() {
+		FlagEnvHinter = withEnvHint
+	}()
+
+	for _, test := range envHintFlagTests {
+		FlagEnvHinter = test.hinter
+		flag := StringFlag{Name: test.name, EnvVar: test.env}
+		output := flag.String()
+		if output != test.expected {
+			t.Errorf("%q does not match %q", output, test.expected)
+		}
+	}
+}
+
 var stringSliceFlagTests = []struct {
 	name     string
 	value    *StringSlice

funcs.go

@@ -26,3 +26,11 @@ type OnUsageErrorFunc func(context *Context, err error, isSubcommand bool) error
 // FlagStringFunc is used by the help generation to display a flag, which is
 // expected to be a single line.
 type FlagStringFunc func(Flag) string
+
+// FlagNamePrefixFunc is used by the default FlagStringFunc to create prefix
+// text for a flag's full name.
+type FlagNamePrefixFunc func(fullName, placeholder string) string
+
+// FlagEnvHintFunc is used by the default FlagStringFunc to annotate flag help
+// with the environment variable details.
+type FlagEnvHintFunc func(envVar, str string) string