How to Unit Test Custom Command Line Flag Validation in Go?

<code class="go">// ... elided for brevity ...</code>

<code class="go">type formatType string

func (f *formatType) String() string {
    return fmt.Sprint(*f)
}

func (f *formatType) Set(value string) error {
    // Validation logic here
}</code>

<code class="go">var typeFlag formatType

func init() {
    // ... elided for brevity ...
    flag.Var(&typeFlag, "format", "Format type")
    flag.Var(&typeFlag, "f", "Format type (shorthand)")
}</code>

<code class="go">func TestCustomFlag(t *testing.T) {
    // Setup test environment
    origArgs := os.Args

    // Define custom flag type
    type myFlag int
    flag.Var((*myFlag)(nil), "customflag", "Custom flag")

    tests := []struct {
        origArgs  []string
        expValue  int
        expOutput string
    }{
        // ... test cases ...
    }

    for _, test := range tests {
        os.Args = test.origArgs
        // Parse flags
        flag.Parse()
        // Check flag value
        if flagValue := flag.Lookup("customflag").Value.(myFlag); flagValue != test.expValue {
            t.Errorf("Expected %v, got %v", test.expValue, flagValue)
        }
        // Restore args
        os.Args = origArgs
    }
}</code>