How to Test Go Code That Relies on Constant Values?

Testing Constants in Go

One common challenge when writing Go programs is testing code that relies on constant values. By default, constants cannot be redefined once defined, making it difficult to simulate different environments during testing.

Problem Scenario

Consider the following code:

<code class="go">package main

import (
    "net/http"
    "net/http/httptest"
)

const baseUrl = "http://google.com"

func main() {
    // Logic that uses baseUrl
}</code>

For testing purposes, you want to set baseUrl to a test server URL. However, redefining const baseUrl in your test file will result in an error:

<code class="go">// in main_test.go
const baseUrl = "test_server_url" // Error: const baseUrl already defined</code>

Solution

To overcome this limitation, you can refactor your code to remove the const and use a function instead. For example:

<code class="go">func GetUrl() string {
    return "http://google.com"
}

func main() {
    // Logic that uses GetUrl()
}</code>

In your test file, you can redefine the function to return the test server URL:

<code class="go">// in main_test.go
func GetUrl() string {
    return "test_server_url"
}</code>

Another Approach

If you prefer to keep the const value, you can create a second function that takes the base URL as a parameter and delegates the actual implementation to the original function:

<code class="go">const baseUrl_ = "http://google.com"

func MyFunc() string {
    // Call other function passing the const value
    return myFuncImpl(baseUrl_)
}

func myFuncImpl(baseUrl string) string {
    // Same implementation that was in your original MyFunc() function
}</code>

By using this approach, you can test the implementation of MyFunc() by testing myFuncImpl(), passing different base URLs for each test case. Additionally, the original MyFunc() function remains safe as it always passes the constant baseUrl_ to myFuncImpl().

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