How to Unit Test Command Line Flag Validation in Go?

How to Unit Test Command Line Flags in Go

Introduction:

Unit testing is crucial for ensuring the reliability and correctness of your code, including command line flag handling. Flag parsing is a common task in Go applications, and it's essential to verify that flags are validated correctly. This article will explore how to unit test command line flags in Go.

Setting Up the Unit Test:

To unit test flag validation, we'll create a custom flag type that checks if a flag's value is within an expected enumeration. Consider the following example code:

<code class="go">var formatType string

const (
    text = "text"
    json = "json"
    hash = "hash"
)

func init() {
    const (
        defaultFormat = "text"
        formatUsage   = "desired output format"
    )

    flag.StringVar(&formatType, "format", defaultFormat, formatUsage)
    flag.StringVar(&formatType, "f", defaultFormat, formatUsage+" (shorthand)")
}</code>

Custom Flag Validation:

We can define a custom flag.Value type to validate the format flag:

<code class="go">type formatType string

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

func (f *formatType) Set(value string) error {
    if len(*f) > 0 && *f != "text" {
        return errors.New("format flag already set")
    }
    if value != "text" && value != "json" && value != "hash" {
        return errors.New("Invalid Format Type")
    }
    *f = formatType(value)
    return nil
}</code>

Creating the Unit Test:

Now, let's write a unit test that verifies the custom flag validation:

<code class="go">func TestValidateFormatFlag(t *testing.T) {
    testCases := []struct {
        input    string
        expected error
    }{
        {"text", nil},
        {"json", nil},
        {"hash", nil},
        {"invalid", errors.New("Invalid Format Type")},
    }

    for _, tc := range testCases {
        args := []string{"-format", tc.input}
        os.Args = args

        typeFlag = "text"
        flag.CommandLine = flag.NewFlagSet("test", flag.ContinueOnError)
        flag.Var(&typeFlag, "format", "Format type. Must be \"text\", \"json\" or \"hash\". Defaults to \"text\".")
        if err := flag.CommandLine.Parse(args[1:]); err != nil {
            t.Errorf("Error parsing flags: %v", err)
        }

        if tc.expected != nil && err == nil {
            t.Errorf("Expected error: %v, got nil", tc.expected)
        } else if tc.expected == nil && err != nil {
            t.Errorf("Unexpected error: %v", err)
        }
    }
}</code>

In this test, we set up a series of test cases with different inputs and expected errors. We mock the command line arguments using os.Args and parse the flags using a custom FlagSet. By comparing the actual error with the expected error for each test case, we ensure that the flag validation works as intended.

Tips for Running Flag Tests:

• Create separate unit tests for each flag validation requirement.
• Use a mock framework like testify to simplify test setup and assertions.
• Avoid using real command line parsing in unit tests to ensure isolation.

Conclusion:

By implementing custom flag types and writing unit tests, you can effectively validate command line flags in Go. This practice helps maintain the reliability and robustness of your applications.

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