How to Effectively Test Argument Passing in Golang Functions?

Testing the Passing of Arguments in Golang

In Golang, passing arguments to a function is a common way to provide inputs. Testing the correctness of argument passing is crucial to ensure the reliability of your code.

The Challenge

Consider the following code snippet:

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
}

When passing the argument -user=bla to this program, it correctly prints "Your username is 'bla'." The challenge is to create a test that verifies this behavior without having to manually build and run the code each time.

The Attempt

One might尝试使用 os.Args to provide the argument in a 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)
    }
}

However, when running this test, the result is:

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

The Solution

The issue is that os.Args is a global variable, so the initial value of ["args"] is overwritten by the test, affecting other parts of the program. To resolve this, we can preserve the original value and restore it after the test:

oldArgs := os.Args
defer func() { os.Args = oldArgs }()
os.Args = []string{"cmd", "-user=bla"}

// ...

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