I'm trying to test one of my package to reach 100%.
However, I can't find how I can do this without being "against the system" (functions pointers, etc.).
I tried to do something similar to this, but I can't reach 100% because of "real" functions :
var fs fileSystem = osFS{}
type fileSystem interface {
Open(name string) (file, error)
Stat(name string) (os.FileInfo, error)
}
type file interface {
io.Closer
io.Reader
io.ReaderAt
io.Seeker
Stat() (os.FileInfo, error)
}
// osFS implements fileSystem using the local disk.
type osFS struct{}
func (osFS) Open(name string) (file, error) { return os.Open(name) }
func (osFS) Stat(name string) (os.FileInfo, error) { return os.Stat(name) }
(From https://talks.golang.org/2012/10things.slide#8)
Does someone would have a suggestion ? :)
Thanks !
I attempted to do same thing, just to try it. I achieved it by referencing all system file calls as interfaces and having method accept an interface. Then if no interface was provided the system method was used. I am brand new to Go, so I'm not sure if it violates best practices or not.
import "io/ioutil"
type ReadFile func (string) ([]byte, error)
type FileLoader interface {
LoadPath(path string) []byte
}
// Initializes a LocalFileSystemLoader with default ioutil.ReadFile
// as the method to read file. Optionally allows caller to provide
// their own ReadFile for testing.
func NewLocalFileSystemLoader(rffn ReadFile) *localFileSystemLoader{
var rf ReadFile = ioutil.ReadFile
if rffn != nil {
rf = rffn
}
return &localFileSystemLoader{
ReadFileFn: rf}
}
type localFileSystemLoader struct {
ReadFileFn ReadFile
}
func (lfsl *localFileSystemLoader) LoadPath(path string) []byte {
dat, err := lfsl.ReadFileFn(path)
if err != nil {
panic(err)
}
return dat
}
Related
For some reason, I cannot seem to get ioutil.ReadAll(res.Body), where res is the *http.Response returned by res, err := hc.Do(redirectRequest) (for hc http.Client, redirectRequest *http.Request).
Testing strategy thus far
Any time I see hc.Do or http.Request in the SUT, my instinct is to spin up a fake server and point the appropriate application states to it. Such a server, for this test, looks like this :
badServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
// some stuff
w.Write([some bad bytes])
}))
defer badServer.Close()
I don't seem to have a way to control res.Body, which is literally the only thing keeping me from 100% test completion against the func this is all in.
I tried, in the errorThrowingServer's handler func, setting r.Body to a stub io.ReadCloser that throws an error when Read() is called, but that doesn't effect res.
You can mock the body. Basically body is an io.ReadCloser interface so, you can do something like this:
import (
"github.com/stretchr/testify/assert"
"github.com/stretchr/testify/mock"
)
type mockReadCloser struct {
mock.Mock
}
func (m *mockReadCloser) Read(p []byte) (n int, err error) {
args := m.Called(p)
return args.Int(0), args.Error(1)
}
func (m *mockReadCloser) Close() error {
args := m.Called()
return args.Error(0)
}
func TestTestingSomethingWithBodyError(t *testing.T) {
mockReadCloser := mockReadCloser{}
// if Read is called, it will return error
mockReadCloser.On("Read", mock.AnythingOfType("[]uint8")).Return(0, fmt.Errorf("error reading"))
// if Close is called, it will return error
mockReadCloser.On("Close").Return(fmt.Errorf("error closing"))
request := &http.Request{
// pass the mock address
Body: &mockReadCloser,
}
expected := "what you expected"
result := YourMethod(request)
assert.Equal(t, expected, result)
mockReadCloser.AssertExpectations(t)
}
To stop reading you can use:
mockReadCloser.On("Read", mock.AnythingOfType("[]uint8")).Return(0, io.EOF).Once()
As far as I could find perusing the source files for all of the working parts, the only way to get http.Response.Body.Read() to fail is commented here:
https://golang.org/src/net/http/response.go#L53
The response body is streamed on demand as the Body field is read. If
the network connection fails or the server terminates the response,
Body.Read calls return an error.
Or there is the possibility in ioutil.ReadAll() for it to return bytes.ErrTooLarge here:
https://golang.org/src/io/ioutil/ioutil.go#L20
If the buffer overflows, we will get bytes.ErrTooLarge. Return that as
an error. Any other panic remains.
There's a method in the code under test, that simply tries to get database connection, or returns error if unable to.
It, and the structs involved are defined as follows:
type DatabaseContext struct {
Context
Database DatabaseSt
}
// //GetInfo Returns the context.
// func (c *DatabaseContext) GetInfo() *Context {
// return &c.Context
// }
//GetDB Gets the database connection from the connection string.
func (c *DatabaseContext) GetDB() (*sql.DB, *errors.ErrorSt) {
var errSt *errors.ErrorSt
if c.Database.dbConnection == nil {
c.Database.dbConnection, errSt = c.openDB()
if errSt != nil {
return nil, errSt
}
c.Database.dbConnection.SetMaxOpenConns(50)
}
return c.Database.dbConnection, nil
}
The other methods, in the same file, which it may hit, are as follows:
//openDB opens the database with the connection string.
func (c *DatabaseContext) openDB() (*sql.DB, *errors.ErrorSt) {
if c.Database.DBConnectionStr == "" {
c.GetDatabase()
}
return db.OpenConnection(c.Database.DBConnectionStr, c.Database.InterpolateParams)
}
//CloseDB Closes the database.
func (c *DatabaseContext) CloseDB() {
if c.Database.dbConnection != nil {
c.Database.dbConnection.Close()
}
}
//SetDatabaseString Sets the database string into the session.
func (c *DatabaseContext) SetDatabaseString(str string) {
c.Database.DBConnectionStr = str
i := strings.Index(str, ")/") + 2
c.Database.DBName = str[i:]
c.SetDatabase()
}
//GetDatabaseString Gets the database string from the session.
func (c *DatabaseContext) GetDatabase() {
if dbIntf := c.GetFromSession("Database"); dbIntf != nil {
c.Database = dbIntf.(DatabaseSt)
}
}
//SetDatabaseString Sets the database string into the session.
func (c *DatabaseContext) SetDatabase() {
c.SetToSession("Database", c.Database)
}
Fortunately, DatabaseContext implements DatabaseContextIntf, which I want to use for testing. My instinct is to straight up mock DatabaseContext, but that won't work because it's not an interface (in Golang, you can only mock interfaces).
How would I go about testing this, without hitting a real database, which can fail beyond my control (thus creating false fails in the test)?
UPDATE My question differs from the suspected duplicate as their question is about database entries, and not connections. The flagged duplicate refers to this library as the answer, however, there is no method in it to return a "connection" that is nil, for the sake of the test. The best it has is New which creates a test double connection, and there's no way to control the state of the returned value (I need it to be nil in one test ("No Connection") and non-nil in another ("Sanity Test"))
I ended up making the package of the test the same as that of the code under test (this allows the test generator in Visual Studio Code to place the generated test right in the test file, and not get confused, as well as give me access to unexported fields, which I used), and just straight up made a fake DatabaseContext
My test case looks like this:
t.Run("SanityTest", func(t *testing.T) {
c := new(DatabaseContext)
assert.Nil(t, c.Database.dbConnection)
database, err := c.GetDB()
defer database.Close()
assert.NotNil(t, database)
if !assert.Nil(t, err) {
t.Error(err.ToString(false))
}
})
How do I fill os.Stdin in my test for a function that reads from it using a scanner?
I request a user command line input via a scanner using following function:
func userInput() error {
scanner := bufio.NewScanner(os.Stdin)
println("What is your name?")
scanner.Scan()
username = scanner.Text()
/* ... */
}
Now how do I test this case and simulate a user input?
Following example does not work. Stdin is still empty.
func TestUserInput(t *testing.T) {
var file *os.File
file.Write([]byte("Tom"))
os.Stdin = file
err := userInput()
/* ... */
}
Mocking os.Stdin
You're on the right track that os.Stdin is a variable (of type *os.File) which you can modify, you can assign a new value to it in tests.
Simplest is to create a temporary file with the content you want to simulate as the input on os.Stdin. To create a temp file, use ioutil.TempFile(). Then write the content into it, and seek back to the beginning of the file. Now you can set it as os.Stdin and perform your tests. Don't forget to cleanup the temp file.
I modified your userInput() to this:
func userInput() error {
scanner := bufio.NewScanner(os.Stdin)
fmt.Println("What is your name?")
var username string
if scanner.Scan() {
username = scanner.Text()
}
if err := scanner.Err(); err != nil {
return err
}
fmt.Println("Entered:", username)
return nil
}
And this is how you can test it:
func TestUserInput(t *testing.T) {
content := []byte("Tom")
tmpfile, err := ioutil.TempFile("", "example")
if err != nil {
log.Fatal(err)
}
defer os.Remove(tmpfile.Name()) // clean up
if _, err := tmpfile.Write(content); err != nil {
log.Fatal(err)
}
if _, err := tmpfile.Seek(0, 0); err != nil {
log.Fatal(err)
}
oldStdin := os.Stdin
defer func() { os.Stdin = oldStdin }() // Restore original Stdin
os.Stdin = tmpfile
if err := userInput(); err != nil {
t.Errorf("userInput failed: %v", err)
}
if err := tmpfile.Close(); err != nil {
log.Fatal(err)
}
}
Running the test, we see an output:
What is your name?
Entered: Tom
PASS
Also see related question about mocking the file system: Example code for testing the filesystem in Golang
The easy, preferred way
Also note that you can refactor userInput() to not read from os.Stdin, but instead it could receive an io.Reader to read from. This would make it more robust and a lot easier to test.
In your app you can simply pass os.Stdin to it, and in tests you can pass any io.Reader to it created / prepared in the tests, e.g. using strings.NewReader(), bytes.NewBuffer() or bytes.NewBufferString().
os.Pipe()
Instead of messing with the actual file system and doing writes and reads to and from real files on a storage device, the simplest solution is using os.Pipe().
Example
The code of your userInput() does have to be adjusted, and #icza's solution would indeed do for that purpose. But the test itself should be something more like this:
func Test_userInput(t *testing.T) {
input := []byte("Alice")
r, w, err := os.Pipe()
if err != nil {
t.Fatal(err)
}
_, err = w.Write(input)
if err != nil {
t.Error(err)
}
w.Close()
// Restore stdin right after the test.
defer func(v *os.File) { os.Stdin = v }(os.Stdin)
os.Stdin = r
if err = userInput(); err != nil {
t.Fatalf("userInput: %v", err)
}
}
Details
There are several important points about this code:
Always close your w stream when you're done writing. Many utilities rely on an io.EOF returned by a Read() call to know that no more data is coming, and the bufio.Scanner is no exception. If you don't close the stream, your scanner.Scan() call will never return, but keep looping internally and waiting for more input until the program is terminated forcefully (as when the test times out).
The pipe buffer capacity varies from system to system, as discussed at length in a post in the Unix & Linux Stack Exchange, so if the size of your simulated input could exceed that, you should wrap your write(s) in a goroutine like so:
//...
go func() {
_, err = w.Write(input)
if err != nil {
t.Error(err)
}
w.Close()
}()
//...
This prevents a deadlock when the pipe is full and writes have to wait for it to start emptying, but the code that's supposed to be reading from and emptying the pipe (userInput() in this case) is not starting, because of writing not being over yet.
A test should also verify that errors are handled properly, in this case, returned by userInput(). This means you'd have to figure out a way to make the scanner.Err() call return an error in a test. One approach could be closing the r stream it was supposed to be reading, before it has had the chance.
Such a test would look almost identical to the nominal case, only you don't write anything at the w end of the pipe, just close the r end, and you actually expect and want userInput() to return an error. And when you have two or more tests of the same function that are almost identical, it is often a good time to implement them as a single table driven test. See Go playground for an example.
io.Reader
The example of userInput() is trivial enough that you could (and should) refactor it and similar cases to read from an io.Reader, just like #icza suggests (see the playground).
You should always strive to rely on some form of dependency injection instead of global state (os.Stdin, in this case, is a global variable in the os package), as that gives more control to the calling code to determine how a called piece of code behaves, which is essential to unit testing, and facilitates better code reuse in general.
Return of os.Pipe()
There may also be cases when you can't really alter a function to take injected dependencies, as when you have to test the main() function of a Go executable. Altering the global state in the test (and hoping that you can properly restore it by the end not to affect subsequent tests) is your only option then. This is where we come back to os.Pipe()
When testing main(), do use os.Pipe() to simulate input to stdin (unless you already hava a file prepared for the purpose) and to capture the output of stdout and stderr (see the playground for an example of the latter).
Implementation of #icza's easy, preferred way:
Also note that you can refactor userInput() to not read from os.Stdin,
but instead it could receive an io.Reader to read from. This would
make it more robust and a lot easier to test.
In your app you can simply pass os.Stdin to it, and in tests you can
pass any io.Reader to it created / prepared in the tests, e.g. using
strings.NewReader(), bytes.NewBuffer() or bytes.NewBufferString().
hello.go
package main
import (
"bufio"
"fmt"
"os"
"io"
)
func userInput(reader io.Reader) error {
scanner := bufio.NewScanner(reader)
var username string
fmt.Println("What is your name?")
if scanner.Scan() {
username = scanner.Text()
}
if scanner.Err() != nil {
return scanner.Err()
}
fmt.Println("Hello", username)
return nil
}
func main() {
userInput(os.Stdin)
}
hello_test.go
package main
import (
"bytes"
"io"
"strings"
"testing"
)
func TestUserInputWithStringsNewReader(t *testing.T) {
input := "Tom"
var reader io.Reader = strings.NewReader(input)
err := userInput(reader)
if err != nil {
t.Errorf("Failed to read from strings.NewReader: %w", err)
}
}
func TestUserInputWithBytesNewBuffer(t *testing.T) {
input := "Tom"
var reader io.Reader = bytes.NewBuffer([]byte(input))
err := userInput(reader)
if err != nil {
t.Errorf("Failed to read from bytes.NewBuffer: %w", err)
}
}
func TestUserInputWithBytesNewBufferString(t *testing.T) {
input := "Tom"
var reader io.Reader = bytes.NewBufferString(input)
err := userInput(reader)
if err != nil {
t.Errorf("Failed to read from bytes.NewBufferString: %w", err)
}
}
Running the program:
go run hello.go
What is your name?
Tom
Hello Tom
Running the test:
go test hello_test.go hello.go -v
=== RUN TestUserInputWithStringsNewReader
What is your name?
Hello Tom
--- PASS: TestUserInputWithStringsNewReader (0.00s)
=== RUN TestUserInputWithBytesNewBuffer
What is your name?
Hello Tom
--- PASS: TestUserInputWithBytesNewBuffer (0.00s)
=== RUN TestUserInputWithBytesNewBufferString
What is your name?
Hello Tom
--- PASS: TestUserInputWithBytesNewBufferString (0.00s)
PASS
ok command-line-arguments 0.141s
You can use *bufio.Scanner to abstract io.Stdin and io.Writer to abstract io.Stdout while passing them as dependencies to your struct, see
Gist: https://gist.github.com/antonzhukov/2a6749f780b24f38b08c9916caa96663 and
Playground: https://play.golang.org/p/BZMqpACupSc
How do I fill os.Stdin in my test for a function that reads from it using a scanner?
I request a user command line input via a scanner using following function:
func userInput() error {
scanner := bufio.NewScanner(os.Stdin)
println("What is your name?")
scanner.Scan()
username = scanner.Text()
/* ... */
}
Now how do I test this case and simulate a user input?
Following example does not work. Stdin is still empty.
func TestUserInput(t *testing.T) {
var file *os.File
file.Write([]byte("Tom"))
os.Stdin = file
err := userInput()
/* ... */
}
Mocking os.Stdin
You're on the right track that os.Stdin is a variable (of type *os.File) which you can modify, you can assign a new value to it in tests.
Simplest is to create a temporary file with the content you want to simulate as the input on os.Stdin. To create a temp file, use ioutil.TempFile(). Then write the content into it, and seek back to the beginning of the file. Now you can set it as os.Stdin and perform your tests. Don't forget to cleanup the temp file.
I modified your userInput() to this:
func userInput() error {
scanner := bufio.NewScanner(os.Stdin)
fmt.Println("What is your name?")
var username string
if scanner.Scan() {
username = scanner.Text()
}
if err := scanner.Err(); err != nil {
return err
}
fmt.Println("Entered:", username)
return nil
}
And this is how you can test it:
func TestUserInput(t *testing.T) {
content := []byte("Tom")
tmpfile, err := ioutil.TempFile("", "example")
if err != nil {
log.Fatal(err)
}
defer os.Remove(tmpfile.Name()) // clean up
if _, err := tmpfile.Write(content); err != nil {
log.Fatal(err)
}
if _, err := tmpfile.Seek(0, 0); err != nil {
log.Fatal(err)
}
oldStdin := os.Stdin
defer func() { os.Stdin = oldStdin }() // Restore original Stdin
os.Stdin = tmpfile
if err := userInput(); err != nil {
t.Errorf("userInput failed: %v", err)
}
if err := tmpfile.Close(); err != nil {
log.Fatal(err)
}
}
Running the test, we see an output:
What is your name?
Entered: Tom
PASS
Also see related question about mocking the file system: Example code for testing the filesystem in Golang
The easy, preferred way
Also note that you can refactor userInput() to not read from os.Stdin, but instead it could receive an io.Reader to read from. This would make it more robust and a lot easier to test.
In your app you can simply pass os.Stdin to it, and in tests you can pass any io.Reader to it created / prepared in the tests, e.g. using strings.NewReader(), bytes.NewBuffer() or bytes.NewBufferString().
os.Pipe()
Instead of messing with the actual file system and doing writes and reads to and from real files on a storage device, the simplest solution is using os.Pipe().
Example
The code of your userInput() does have to be adjusted, and #icza's solution would indeed do for that purpose. But the test itself should be something more like this:
func Test_userInput(t *testing.T) {
input := []byte("Alice")
r, w, err := os.Pipe()
if err != nil {
t.Fatal(err)
}
_, err = w.Write(input)
if err != nil {
t.Error(err)
}
w.Close()
// Restore stdin right after the test.
defer func(v *os.File) { os.Stdin = v }(os.Stdin)
os.Stdin = r
if err = userInput(); err != nil {
t.Fatalf("userInput: %v", err)
}
}
Details
There are several important points about this code:
Always close your w stream when you're done writing. Many utilities rely on an io.EOF returned by a Read() call to know that no more data is coming, and the bufio.Scanner is no exception. If you don't close the stream, your scanner.Scan() call will never return, but keep looping internally and waiting for more input until the program is terminated forcefully (as when the test times out).
The pipe buffer capacity varies from system to system, as discussed at length in a post in the Unix & Linux Stack Exchange, so if the size of your simulated input could exceed that, you should wrap your write(s) in a goroutine like so:
//...
go func() {
_, err = w.Write(input)
if err != nil {
t.Error(err)
}
w.Close()
}()
//...
This prevents a deadlock when the pipe is full and writes have to wait for it to start emptying, but the code that's supposed to be reading from and emptying the pipe (userInput() in this case) is not starting, because of writing not being over yet.
A test should also verify that errors are handled properly, in this case, returned by userInput(). This means you'd have to figure out a way to make the scanner.Err() call return an error in a test. One approach could be closing the r stream it was supposed to be reading, before it has had the chance.
Such a test would look almost identical to the nominal case, only you don't write anything at the w end of the pipe, just close the r end, and you actually expect and want userInput() to return an error. And when you have two or more tests of the same function that are almost identical, it is often a good time to implement them as a single table driven test. See Go playground for an example.
io.Reader
The example of userInput() is trivial enough that you could (and should) refactor it and similar cases to read from an io.Reader, just like #icza suggests (see the playground).
You should always strive to rely on some form of dependency injection instead of global state (os.Stdin, in this case, is a global variable in the os package), as that gives more control to the calling code to determine how a called piece of code behaves, which is essential to unit testing, and facilitates better code reuse in general.
Return of os.Pipe()
There may also be cases when you can't really alter a function to take injected dependencies, as when you have to test the main() function of a Go executable. Altering the global state in the test (and hoping that you can properly restore it by the end not to affect subsequent tests) is your only option then. This is where we come back to os.Pipe()
When testing main(), do use os.Pipe() to simulate input to stdin (unless you already hava a file prepared for the purpose) and to capture the output of stdout and stderr (see the playground for an example of the latter).
Implementation of #icza's easy, preferred way:
Also note that you can refactor userInput() to not read from os.Stdin,
but instead it could receive an io.Reader to read from. This would
make it more robust and a lot easier to test.
In your app you can simply pass os.Stdin to it, and in tests you can
pass any io.Reader to it created / prepared in the tests, e.g. using
strings.NewReader(), bytes.NewBuffer() or bytes.NewBufferString().
hello.go
package main
import (
"bufio"
"fmt"
"os"
"io"
)
func userInput(reader io.Reader) error {
scanner := bufio.NewScanner(reader)
var username string
fmt.Println("What is your name?")
if scanner.Scan() {
username = scanner.Text()
}
if scanner.Err() != nil {
return scanner.Err()
}
fmt.Println("Hello", username)
return nil
}
func main() {
userInput(os.Stdin)
}
hello_test.go
package main
import (
"bytes"
"io"
"strings"
"testing"
)
func TestUserInputWithStringsNewReader(t *testing.T) {
input := "Tom"
var reader io.Reader = strings.NewReader(input)
err := userInput(reader)
if err != nil {
t.Errorf("Failed to read from strings.NewReader: %w", err)
}
}
func TestUserInputWithBytesNewBuffer(t *testing.T) {
input := "Tom"
var reader io.Reader = bytes.NewBuffer([]byte(input))
err := userInput(reader)
if err != nil {
t.Errorf("Failed to read from bytes.NewBuffer: %w", err)
}
}
func TestUserInputWithBytesNewBufferString(t *testing.T) {
input := "Tom"
var reader io.Reader = bytes.NewBufferString(input)
err := userInput(reader)
if err != nil {
t.Errorf("Failed to read from bytes.NewBufferString: %w", err)
}
}
Running the program:
go run hello.go
What is your name?
Tom
Hello Tom
Running the test:
go test hello_test.go hello.go -v
=== RUN TestUserInputWithStringsNewReader
What is your name?
Hello Tom
--- PASS: TestUserInputWithStringsNewReader (0.00s)
=== RUN TestUserInputWithBytesNewBuffer
What is your name?
Hello Tom
--- PASS: TestUserInputWithBytesNewBuffer (0.00s)
=== RUN TestUserInputWithBytesNewBufferString
What is your name?
Hello Tom
--- PASS: TestUserInputWithBytesNewBufferString (0.00s)
PASS
ok command-line-arguments 0.141s
You can use *bufio.Scanner to abstract io.Stdin and io.Writer to abstract io.Stdout while passing them as dependencies to your struct, see
Gist: https://gist.github.com/antonzhukov/2a6749f780b24f38b08c9916caa96663 and
Playground: https://play.golang.org/p/BZMqpACupSc
How do I fill os.Stdin in my test for a function that reads from it using a scanner?
I request a user command line input via a scanner using following function:
func userInput() error {
scanner := bufio.NewScanner(os.Stdin)
println("What is your name?")
scanner.Scan()
username = scanner.Text()
/* ... */
}
Now how do I test this case and simulate a user input?
Following example does not work. Stdin is still empty.
func TestUserInput(t *testing.T) {
var file *os.File
file.Write([]byte("Tom"))
os.Stdin = file
err := userInput()
/* ... */
}
Mocking os.Stdin
You're on the right track that os.Stdin is a variable (of type *os.File) which you can modify, you can assign a new value to it in tests.
Simplest is to create a temporary file with the content you want to simulate as the input on os.Stdin. To create a temp file, use ioutil.TempFile(). Then write the content into it, and seek back to the beginning of the file. Now you can set it as os.Stdin and perform your tests. Don't forget to cleanup the temp file.
I modified your userInput() to this:
func userInput() error {
scanner := bufio.NewScanner(os.Stdin)
fmt.Println("What is your name?")
var username string
if scanner.Scan() {
username = scanner.Text()
}
if err := scanner.Err(); err != nil {
return err
}
fmt.Println("Entered:", username)
return nil
}
And this is how you can test it:
func TestUserInput(t *testing.T) {
content := []byte("Tom")
tmpfile, err := ioutil.TempFile("", "example")
if err != nil {
log.Fatal(err)
}
defer os.Remove(tmpfile.Name()) // clean up
if _, err := tmpfile.Write(content); err != nil {
log.Fatal(err)
}
if _, err := tmpfile.Seek(0, 0); err != nil {
log.Fatal(err)
}
oldStdin := os.Stdin
defer func() { os.Stdin = oldStdin }() // Restore original Stdin
os.Stdin = tmpfile
if err := userInput(); err != nil {
t.Errorf("userInput failed: %v", err)
}
if err := tmpfile.Close(); err != nil {
log.Fatal(err)
}
}
Running the test, we see an output:
What is your name?
Entered: Tom
PASS
Also see related question about mocking the file system: Example code for testing the filesystem in Golang
The easy, preferred way
Also note that you can refactor userInput() to not read from os.Stdin, but instead it could receive an io.Reader to read from. This would make it more robust and a lot easier to test.
In your app you can simply pass os.Stdin to it, and in tests you can pass any io.Reader to it created / prepared in the tests, e.g. using strings.NewReader(), bytes.NewBuffer() or bytes.NewBufferString().
os.Pipe()
Instead of messing with the actual file system and doing writes and reads to and from real files on a storage device, the simplest solution is using os.Pipe().
Example
The code of your userInput() does have to be adjusted, and #icza's solution would indeed do for that purpose. But the test itself should be something more like this:
func Test_userInput(t *testing.T) {
input := []byte("Alice")
r, w, err := os.Pipe()
if err != nil {
t.Fatal(err)
}
_, err = w.Write(input)
if err != nil {
t.Error(err)
}
w.Close()
// Restore stdin right after the test.
defer func(v *os.File) { os.Stdin = v }(os.Stdin)
os.Stdin = r
if err = userInput(); err != nil {
t.Fatalf("userInput: %v", err)
}
}
Details
There are several important points about this code:
Always close your w stream when you're done writing. Many utilities rely on an io.EOF returned by a Read() call to know that no more data is coming, and the bufio.Scanner is no exception. If you don't close the stream, your scanner.Scan() call will never return, but keep looping internally and waiting for more input until the program is terminated forcefully (as when the test times out).
The pipe buffer capacity varies from system to system, as discussed at length in a post in the Unix & Linux Stack Exchange, so if the size of your simulated input could exceed that, you should wrap your write(s) in a goroutine like so:
//...
go func() {
_, err = w.Write(input)
if err != nil {
t.Error(err)
}
w.Close()
}()
//...
This prevents a deadlock when the pipe is full and writes have to wait for it to start emptying, but the code that's supposed to be reading from and emptying the pipe (userInput() in this case) is not starting, because of writing not being over yet.
A test should also verify that errors are handled properly, in this case, returned by userInput(). This means you'd have to figure out a way to make the scanner.Err() call return an error in a test. One approach could be closing the r stream it was supposed to be reading, before it has had the chance.
Such a test would look almost identical to the nominal case, only you don't write anything at the w end of the pipe, just close the r end, and you actually expect and want userInput() to return an error. And when you have two or more tests of the same function that are almost identical, it is often a good time to implement them as a single table driven test. See Go playground for an example.
io.Reader
The example of userInput() is trivial enough that you could (and should) refactor it and similar cases to read from an io.Reader, just like #icza suggests (see the playground).
You should always strive to rely on some form of dependency injection instead of global state (os.Stdin, in this case, is a global variable in the os package), as that gives more control to the calling code to determine how a called piece of code behaves, which is essential to unit testing, and facilitates better code reuse in general.
Return of os.Pipe()
There may also be cases when you can't really alter a function to take injected dependencies, as when you have to test the main() function of a Go executable. Altering the global state in the test (and hoping that you can properly restore it by the end not to affect subsequent tests) is your only option then. This is where we come back to os.Pipe()
When testing main(), do use os.Pipe() to simulate input to stdin (unless you already hava a file prepared for the purpose) and to capture the output of stdout and stderr (see the playground for an example of the latter).
Implementation of #icza's easy, preferred way:
Also note that you can refactor userInput() to not read from os.Stdin,
but instead it could receive an io.Reader to read from. This would
make it more robust and a lot easier to test.
In your app you can simply pass os.Stdin to it, and in tests you can
pass any io.Reader to it created / prepared in the tests, e.g. using
strings.NewReader(), bytes.NewBuffer() or bytes.NewBufferString().
hello.go
package main
import (
"bufio"
"fmt"
"os"
"io"
)
func userInput(reader io.Reader) error {
scanner := bufio.NewScanner(reader)
var username string
fmt.Println("What is your name?")
if scanner.Scan() {
username = scanner.Text()
}
if scanner.Err() != nil {
return scanner.Err()
}
fmt.Println("Hello", username)
return nil
}
func main() {
userInput(os.Stdin)
}
hello_test.go
package main
import (
"bytes"
"io"
"strings"
"testing"
)
func TestUserInputWithStringsNewReader(t *testing.T) {
input := "Tom"
var reader io.Reader = strings.NewReader(input)
err := userInput(reader)
if err != nil {
t.Errorf("Failed to read from strings.NewReader: %w", err)
}
}
func TestUserInputWithBytesNewBuffer(t *testing.T) {
input := "Tom"
var reader io.Reader = bytes.NewBuffer([]byte(input))
err := userInput(reader)
if err != nil {
t.Errorf("Failed to read from bytes.NewBuffer: %w", err)
}
}
func TestUserInputWithBytesNewBufferString(t *testing.T) {
input := "Tom"
var reader io.Reader = bytes.NewBufferString(input)
err := userInput(reader)
if err != nil {
t.Errorf("Failed to read from bytes.NewBufferString: %w", err)
}
}
Running the program:
go run hello.go
What is your name?
Tom
Hello Tom
Running the test:
go test hello_test.go hello.go -v
=== RUN TestUserInputWithStringsNewReader
What is your name?
Hello Tom
--- PASS: TestUserInputWithStringsNewReader (0.00s)
=== RUN TestUserInputWithBytesNewBuffer
What is your name?
Hello Tom
--- PASS: TestUserInputWithBytesNewBuffer (0.00s)
=== RUN TestUserInputWithBytesNewBufferString
What is your name?
Hello Tom
--- PASS: TestUserInputWithBytesNewBufferString (0.00s)
PASS
ok command-line-arguments 0.141s
You can use *bufio.Scanner to abstract io.Stdin and io.Writer to abstract io.Stdout while passing them as dependencies to your struct, see
Gist: https://gist.github.com/antonzhukov/2a6749f780b24f38b08c9916caa96663 and
Playground: https://play.golang.org/p/BZMqpACupSc