How to Solve \'panic: open templates/index.html: no such file or directory\' Errors in Go App Engine Unit Tests?

How to Specify Template Paths for App Engine with Go Unit Testing

When using App Engine with Go's built-in template package, unit tests may encounter issues locating template files. This is because, during local development, the server finds the template files relative to the app root, while unit tests run in a different directory.

The Issue

The unit test panics with the following message: "panic: open templates/index.html: no such file or directory." This indicates that the server cannot find the index.html template file.

Option 1: Change Working Directory

One option is to change the working directory to the app root before calling the code that uses relative paths to the templates. This can be achieved with os.Chdir().

import "os"

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

Option 2: Refactor Code

Another option is to refactor the code that uses relative paths to accept a base path. This base path can be set to the app root during tests, allowing the relative paths to function correctly.

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

In the unit test, the base path can be set to the app root, ensuring that the template files can be located.

func TestPageIndex(t *testing.T) {
    inst, _ := aetest.NewInstance(nil)
    //ignoring the errors for brevity
    defer inst.Close()

    req, _ := inst.NewRequest("GET", "/", nil)
    resp := httptest.NewRecorder()
    pageIndex(resp, req, "../..") // Set base path to app root
}

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