Mock functions in Go

Question

I m puzzled with dependencies  I want to be able to replace some function calls with mock ones  Here s a snippet of my code  func get page url string  string       get dl slot url      defer free dl slot url           resp  err    http Get url      if err    nil   return  quot  quot        defer resp Body Close            contents  err    ioutil ReadAll resp Body      if err    nil   return  quot  quot        return string contents     func downloader         dl slots   make chan bool  DL SLOT AMOUNT     Init the download slot semaphore     content    get page BASE URL      links regexp    regexp MustCompile LIST LINK REGEXP      matches    links regexp FindAllStringSubmatch content  -1      for    match    range matches          go serie dl match 1   match 2            I d like to be able to test downloader   without actually getting a page through http - i e  by mocking either get page  easier since it returns just the page content as a string  or http Get    I found this thread which seems to be about a similar problem  Julian Phillips presents his library  Withmock as a solution  but I m unable to get it to work  Here s the relevant parts of my testing code  which is largely cargo cult code to me  to be honest  import        quot testing quot       quot net http quot     mock      quot code google com p gomock quot        func TestDownloader  t  testing T        ctrl    gomock NewController       defer ctrl Finish       http MOCK   SetController ctrl      http EXPECT   Get BASE URL      downloader          The rest to be written    The test output is following   ERROR  Failed to install   et http   exit status 1 output  can t load package  package  et http  found packages http  chunked go  and main  main mock go  in  var folders z9 ql yn5h550s6shtb9c5sggj40000gn T withmock570825607 path src  et http  Is the Withmock a solution to my testing problem  What should I do to get it to work

User · Answer

If you change your function definition to use a variable instead:

var get_page = func(url string) string {
    ...
}

You can override it in your tests:

func TestDownloader(t *testing.T) {
    get_page = func(url string) string {
        if url != "expected" {
            t.Fatal("good message")
        }
        return "something"
    }
    downloader()
}

Careful though, your other tests might fail if they test the functionality of the function you override!

The Go authors use this pattern in the Go standard library to insert test hooks into code to make things easier to test:

User · Answer

Warning  This might inflate executable file size a little bit and cost a little runtime performance  IMO  this would be better if golang has such feature like macro or function decorator   If you want to mock functions without changing its API  the easiest way is to change the implementation a little bit   func getPage url string  string     if GetPageMock    nil       return GetPageMock             getPage real implementation goes here     func downloader       if GetPageMock    nil       return GetPageMock             getPage real implementation goes here     var GetPageMock func url string  string   nil var DownloaderMock func     nil   This way we can actually mock one function out of the others  For more convenient we can provide such mocking boilerplate      download go func getPage url string  string     if m GetPageMock    nil       return m GetPageMock             getPage real implementation goes here     func downloader       if m GetPageMock    nil       return m GetPageMock             getPage real implementation goes here     type MockHandler struct     GetPage func url string  string   Downloader func      var m  MockHandler   new MockHandler   func Mock handler  MockHandler      m   handler     In test file      download test go func GetPageMock url string  string               func TestDownloader t  testing T      Mock  amp MockHandler      GetPage  GetPageMock             Test implementation goes here     Mock new MockHandler      Reset mocked functions

User · Answer

I m using a slightly different approach where public struct methods implement interfaces but their logic is limited to just wrapping private  unexported  functions which take those interfaces as parameters  This gives you the granularity you would need to mock virtually any dependency and yet have a clean API to use from outside your test suite  To understand this it is imperative to understand that you have access to the unexported methods in your test case  i e  from within your  test go files  so you test those instead of testing the exported ones which have no logic inside beside wrapping  To summarize  test the unexported functions instead of testing the exported ones  Let s make an example  Say that we have a Slack API struct which has two methods   the SendMessage method which sends an HTTP request to a Slack webhook the SendDataSynchronously method which given a slice of strings iterates over them and calls SendMessage for every iteration  So in order to test SendDataSynchronously without making an HTTP request each time we would have to mock SendMessage  right   package main  import        quot fmt quot        URI interface type URI interface       GetURL   string       MessageSender interface type MessageSender interface       SendMessage message string  error       This one is the  quot object quot  that our users will call to use this package functionalities type API struct       baseURL  string     endpoint string       Here we make API implement implicitly the URI interface func  api  API  GetURL   string       return api baseURL   api endpoint       Here we make API implement implicitly the MessageSender interface    Again we re just WRAPPING the sendMessage function here  nothing fancy  func  api  API  SendMessage message string  error       return sendMessage api  message        We want to test this method but it calls SendMessage which makes a real HTTP request     Again we re just WRAPPING the sendDataSynchronously function here  nothing fancy func  api  API  SendDataSynchronously data   string  error       return sendDataSynchronously api  data        this would make a real HTTP request func sendMessage uri URI  message string  error       fmt Println  quot This function won t get called because we will mock it quot       return nil       this is the function we want to test    func sendDataSynchronously sender MessageSender  data   string  error       for    text    range data           err    sender SendMessage text           if err    nil               return err                      return nil       TEST CASE BELOW     Here s our mock which just contains some variables that will be filled for running assertions on them later on type mockedSender struct       err      error     messages   string       We make our mock implement the MessageSender interface so we can test sendDataSynchronously func  sender  mockedSender  SendMessage message string  error          let s store all received messages for later assertions     sender messages   append sender messages  message       return sender err    return error for later assertions    func TestSendsAllMessagesSynchronously         mockedMessages    make   string  0      sender    mockedSender nil  mockedMessages       messagesToSend      string  quot one quot    quot two quot    quot three quot       err    sendDataSynchronously  amp sender  messagesToSend       if err    nil           fmt Println  quot All good here we expect the error to be nil  quot   err             expectedMessages    fmt Sprintf  quot  v quot   messagesToSend      actualMessages    fmt Sprintf  quot  v quot   sender messages       if expectedMessages    actualMessages           fmt Println  quot Actual messages are as expected  quot   actualMessages           func main         TestSendsAllMessagesSynchronously      What I like about this approach is that by looking at the unexported methods you can clearly see what the dependencies are  At the same time the API that you export is a lot cleaner and with less parameters to pass along since the true dependency here is just the parent receiver which is implementing all those interfaces itself  Yet every function is potentially depending only on one part of it  one  maybe two interfaces  which makes refactors a lot easier  It s nice to see how your code is really coupled just by looking at the functions signatures  I think it makes a powerful tool against smelling code  To make things easy I put everything into one file to allow you to run the code in the playground here but I suggest you also check out the full example on GitHub  here is the slack go file and here the slack test go  And here the whole thing

User · Answer

Personally  I don t use gomock  or any mocking framework for that matter  mocking in Go is very easy without it   I would either pass a dependency to the downloader   function as a parameter  or I would make downloader   a method on a type  and the type can hold the get page dependency  Method 1  Pass get page   as a parameter of downloader   type PageGetter func url string  string  func downloader pageGetterFunc PageGetter                   content    pageGetterFunc BASE URL                Main  func get page url string  string                func main         downloader get page     Test  func mock get page url string  string          mock your  get page    function here    func TestDownloader t  testing T        downloader mock get page     Method2  Make download   a method of a type Downloader  If you don t want to pass the dependency as a parameter  you could also make get page   a member of a type  and make download   a method of that type  which can then use get page  type PageGetter func url string  string  type Downloader struct       get page PageGetter    func NewDownloader pg PageGetter   Downloader       return  amp Downloader get page  pg     func  d  Downloader  download                   content    d get page BASE URL               Main  func get page url string  string                func main         d    NewDownloader get page      d download      Test  func mock get page url string  string          mock your  get page    function here    func TestDownloader         d    NewDownloader mock get page      d download

User · Answer

Considering unit test is the domain of this question  highly recommend you to use monkey  This Package make you to mock test without changing your original source code  Compare to other answer  it s more  quot non-intrusive quot   main type AA struct            func  a  AA  OriginalFunc              mock test var a  AA  func NewFunc a  AA              monkey PatchMethod reflect TypeOf a    quot OriginalFunc quot   NewFunc   Bad side is   Reminded by Dave C  This method is unsafe  So don t use it outside of unit test  Is non-idiomatic Go   Good side is   Is non-intrusive  Make you do things without changing the main code  Like Thomas said  Make you change behavior of package  maybe provided by third party  with least code

User · Answer

I would do something like   Main  var getPage   get page func get page       func downloader         dl slots   make chan bool  DL SLOT AMOUNT     Init the download slot semaphore     content    getPage BASE URL      links regexp    regexp MustCompile LIST LINK REGEXP      matches    links regexp FindAllStringSubmatch content  -1      for    match    range matches          go serie dl match 1   match 2             Test  func TestDownloader  t  testing T        origGetPage    getPage     getPage   mock get page     defer func    getPage   origGatePage           The rest to be written       define mock get page and rest of the codes func mock get page         And I would avoid   in golang  Better use camelCase

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