如何测试ioutil.ReadFile和os.Stat？

Question

I have the following function:

func GetDataFromFile(path string) ([]byte, error) {
    _, err := os.Stat(path)
    if err != nil {
        return nil, err
    }
    data, err := ioutil.ReadFile(path)
    if err != nil {
        return nil, err
    }
    return data, nil
}

I want to do tests for functions ioutil.ReadFile and os.Stat(path) when they throw errors. I know that I can create non-exist path for os.Stat(path), but how to test such kind functions without "workarounds" and guessing how functions are working?

Regards.

Answer 1

I agree with abhink here, I would not expect you to test this particular function. But in practice, similar situation happens often.

My best solution is to use a factory to create GetDataFromFile. In this case, you inject the dependencies.

main.go

package main

import (
    "io/ioutil"
    "os"
)

func getDataFromFileFactory(
    stat func(filename string) (os.FileInfo, error),
    readFile func(filename string) ([]byte, error),
) func(path string) ([]byte, error) {

    return func(path string) ([]byte, error) {
        _, err := stat(path)
        if err != nil {
            return nil, err
        }
        data, err := readFile(path)
        if err != nil {
            return nil, err
        }
        return data, nil
    }
}

var GetDataFromFile = getDataFromFileFactory(os.Stat, ioutil.ReadFile)

func main() {}

main_test.go

package main

import (
    "errors"
    "os"
    "testing"
)

func TestGetDataFromFile(t *testing.T) {
    stat := func(filename string) (os.FileInfo, error) {
        return nil, errors.New("err msg")
    }

    readfile := func(filename string) ([]byte, error) {
        t.Error("should not call this function")
        return nil, nil
    }

    getDataFromFile := getDataFromFileFactory(stat, readfile)

    if _, err := getDataFromFile("foo"); err.Error() != "err msg" {
        t.Error("expected an error to be thrown")
    }

}