Why Does My Go Test for Argument Passing Using `os.Args` Fail, and How Can I Fix It?

Testing Argument Passing in Go

Problem:

When writing a test to verify argument passing in Go using os.Args, it results in unexpected behavior where the expected argument is not received by the function.

Code Snippet:

package main

import (
    "flag"
    "fmt"
    "testing"

    "os"
)

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
}

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)
    }
}

Attempt:

The following test fails, indicating that the argument is not being passed correctly:

--- FAIL: TestArgs (0.00s)
    args_test.go:15: Test failed, expected: 'bla', got:  'root'
FAIL
coverage: 87.5% of statements
FAIL    tool    0.008s

Solution:

The issue lies in the assignment of os.Args. The syntax should be:

os.Args = []string{"cmd", "-user=bla"}

Additionally, it's recommended to preserve the original value of os.Args before the test and restore it afterward:

oldArgs := os.Args
defer func() { os.Args = oldArgs }()

This prevents other tests from being affected by the modification of os.Args.

Explanation:

The os.Args slice is a "global variable" that contains the original arguments passed to the program. By assigning a new slice to os.Args, we effectively replace the original arguments. The first element of os.Args is typically the path to the executable. By prepending "cmd" to the argument list, we ensure that the function receives the correct argument value.

Restoring the original value of os.Args after the test is good practice to maintain a consistent environment for other tests.

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