Testing a method that loads JSON config files in Golang

Question

In the Golang project under test, there's a method that loads a JSON config file into a variable. Its code is like this:

// Load the JSON config file
func Load(configFile string, outputObj interface{}) *errors.ErrorSt {
    var err error
    // Read the config file
    jsonBytes, err := ioutil.ReadFile(configFile)
    if err != nil {
        fmt.Println(err.Error())
        return errors.File().AddDetails(err.Error())
    }

    // Parse the config
    if err := json.Unmarshal(jsonBytes, outputObj); err != nil {
        return errors.JSON().AddDetails("Could not parse" + configFile + ": " + err.Error())
    }
    return nil

}

I wish to test it but I don't know if I should create fake JSON file for the test cases, or just mock the whole function. My Java background has me leaning towards the latter.

Exploring that, I found the testify framework I'm using has a package for mocking methods, but what I'm attempting to mock doesn't belong to interfaces (the pitfalls of non-OOP languages!!)

Answer 1

There's a couple ways to do it. It's certainly not unusual to have a sample data file to test loading & parsing a file (you'll find this in places in the standard library). It's also a pretty common practice for a function like this to take in an io.Reader rather than a file path, so that in testing you can just pass in e.g. a bytes.Reader to effectively "mock" the file while testing everything else. Which method to use (or both, if you choose) depends on your use case and design objectives; switching to an io.Reader gives you more flexibility, but only you know if that flexibility has any value in context. If not, just keep a test file along with your tests and read that.