How to Properly Test Go's `flag` Package Argument Parsing in Unit Tests?

How to Test Argument Parsing in Go

Consider the following Go code, which parses command-line arguments using the flag package:

package main

import (
    "flag"
    "fmt"
)

func main() {
    passArguments()
}

func passArguments() string {
    username := flag.String("user", "root", "Username for this server")
    flag.Parse()
    fmt.Printf("Your username is %q.", *username)

    usernameToString := *username
    return usernameToString
}

To test this function, we can pass an argument to the compiled code, as shown below:

./args -user=bla

This should result in the output:

Your username is "bla"

However, if we attempt to test this behavior with the following test:

package main

import (
    "os"
    "testing"
)

func TestArgs(t *testing.T) {
    expected := "bla"
    os.Args = []string{"-user=bla"}

    actual := passArguments()

    if actual != expected {
        t.Errorf("Test failed, expected: '%s', got:  '%s'", expected, actual)
    }
}

We receive an error:

FAIL
coverage: 87.5% of statements
FAIL    tool    0.008s

The reason for this failure is that simply setting os.Args to a new slice does not pass the argument to the function. To resolve this, we need to ensure that the very first value in os.Args is a (path to) executable itself, as demonstrated in this revised test:

func TestArgs(t *testing.T) {
    expected := "bla"
    oldArgs := os.Args
    defer func() { os.Args = oldArgs }()
    os.Args = []string{"cmd", "-user=bla"}

    actual := passArguments()

    if actual != expected {
        t.Errorf("Test failed, expected: '%s', got:  '%s'", expected, actual)
    }
}

By deferring the restoration of os.Args to its original state, we can ensure that the test doesn't interfere with other tests that may be examining the real arguments passed when evoking go test.

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