How to Resolve \'no such file or directory\' Errors When Unit Testing App Engine Templates with Custom Paths?

Testing App Engine Templates with Custom Paths

When using the template package with Go on App Engine, you may encounter an issue with file resolution during unit testing. The unit test fails with the error "open templates/index.html: no such file or directory," indicating that the server cannot locate the template file.

The solution to this issue lies in understanding the difference between the app root (where app.yaml resides) and the current directory when running unit tests. Unit tests are typically run in the folder containing the *_test.go file, which is not the app root. Relative file paths that work correctly during normal app execution will not resolve correctly when running tests.

To fix this issue, you can either:

1. Change the Working Directory to the App Root:

Use os.Chdir() to navigate to the app root directory in your test file, which is typically 2 levels up from the test file location. For example:

func init() {
    if err := os.Chdir("../.."); err != nil {
        panic(err)
    }
}

Note that this must be done in the init() function or called explicitly in the test method.

2. Refactor Code:

Instead of using relative file paths, refactor the code to pass the app root as a parameter or variable. This allows you to specify the base path for relative file resolution during unit testing independently from the current directory.

// Package scope variable for passing the app root
var appRoot string

func pageIndex(w http.ResponseWriter, r *http.Request) {
    tpls := append([]string{"templates/index.html"}, templates...)
    tpl := template.Must(template.ParseFiles(append([]string{appRoot}, tpls...)...))
    // ...
}

// Function to initialize the app root before running tests
func TestMain(m *testing.M) {
    // Set appRoot to the absolute path of the app root
    appRoot = "../.."
    // ...
    os.Exit(m.Run())
}

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