How to mock functions in Go ?

[Nitish Saboo]

Hi,

How can we mock a function in Go like os.Hostname() or os.Getwd() ?

Any framework or Library that we can use for mocking the function calls ?

Thanks,

Nitish

[Kyle Butz]

You'll want to look into gomock and mockgen.  To get started mocking, you'll need an interface, so maybe writing a receiver function encapsulating os.Hostname(), then consuming that with an interface.

type OsClient struct {}

func (o *OsClient) GetOSHostname() (string, error) { return os.Hostname() }

type HostnameGetter interface { GetOSHostname() (string, error) }

Gomock repo w/ documentation: https://github.com/golang/mock

Nice intros to gomock:

- https://blog.codecentric.de/en/2017/08/gomock-tutorial/

- https://www.philosophicalhacker.com/post/getting-started-with-gomock/

Note to the mods: I accidentally posted this from the wrong, unverified account first

[Robert Engels]

That brings up an interesting idea. A ‘package replace’ during compile. So you request that the os package is replaced by osmock etc. 

This would allow easy general reuse mocking frameworks without changing the stdlib and all user code. 

On Aug 1, 2019, at 8:38 AM, kyle...@sezzle.com wrote:

You'll want to check out `gomock` and `mockgen`. You'll need to write an interface to be able to mock, so you may need to write a receiver struct to encapsulate os.Hostname() or whatever you need to mock, then mock your HostnameGetter (surely a better name than that), maybe?

type OsClient struct {}

(o *OsClient) GetHostname() (string, error) { return os.Hostname() }

type HostnameGetter interface { GetHostname() (string, error)}

Here is repo w/ documentation: https://github.com/golang/mock

And here are a couple other nice introductions:

- https://blog.codecentric.de/en/2017/08/gomock-tutorial/

- https://www.philosophicalhacker.com/post/getting-started-with-gomock/

On Thursday, August 1, 2019 at 7:09:54 AM UTC-5, Nitish Saboo wrote:

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[Shulhan]

I usually does something like these,

var getHostname = os.Hostname

and in test file,

getHostname = func() {
return "something", nil // or a variable
}

--
{ "github":"github.com/shuLhan", "site":"kilabit.info" }

[Edward Muller]

This can also be done without an interface: https://play.golang.org/p/fgRX2nXIxn0

In ^ example “Thing” would be the type you are working on, with functional options (https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis), and defaults to using os.Hostname.

PS: Use whichever method works for you, this is just my opinions.

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[Andrey Tcherepanov]

what if you need only a subpart of the package to be mocked ?

On Thursday, August 1, 2019 at 5:29:30 PM UTC+3, Robert Engels wrote:

That brings up an interesting idea. A ‘package replace’ during compile. So you request that the os package is replaced by osmock etc. 

This would allow easy general reuse mocking frameworks without changing the stdlib and all user code. 

On Aug 1, 2019, at 8:38 AM, kyle...@sezzle.com wrote:

You'll want to check out `gomock` and `mockgen`. You'll need to write an interface to be able to mock, so you may need to write a receiver struct to encapsulate os.Hostname() or whatever you need to mock, then mock your HostnameGetter (surely a better name than that), maybe?

type OsClient struct {}

(o *OsClient) GetHostname() (string, error) { return os.Hostname() }

type HostnameGetter interface { GetHostname() (string, error)}

Here is repo w/ documentation: https://github.com/golang/mock

And here are a couple other nice introductions:

- https://blog.codecentric.de/en/2017/08/gomock-tutorial/

- https://www.philosophicalhacker.com/post/getting-started-with-gomock/

On Thursday, August 1, 2019 at 7:09:54 AM UTC-5, Nitish Saboo wrote:

Hi,

How can we mock a function in Go like os.Hostname() or os.Getwd() ?

Any framework or Library that we can use for mocking the function calls ?

Thanks,

Nitish

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[Robert Engels]

In an ideal case got exported functions it would defer to the original. 

Still I think the package might have too many responsibilities if that’s the case. 

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[Robert Engels]

Similarly the fact that you need to mock just os.Hostname and you know that means you are probably testing implementation details which I wouldn’t do. As I’ve pointed out before most of the Go stdlib tests are black box in a different test package. I think you’ll create better more resilient tests that way. 

To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/29CF1F67-E62C-410D-8DCF-9DFD75A10B62%40ix.netcom.com.

[Wojciech S. Czarnecki]

On Sat, 3 Aug 2019 22:35:44 -0700 (PDT)
Andrey Tcherepanov <xnow4f...@sneakemail.com> wrote:

> what if you need only a subpart of the package to be mocked ?

import smthorig "smth"

type smthT struct{
//functions
f1 func signature
f2 func signature
}

// functions
func mof1 (in...) (out...) { /* here your mock goes */ }
func mof2 (in...) (out...) { return smthorig(in...) }

var smth = smthT{ f1: mof1, f2: mof2 }

// methods
func (o smth) m1 (in...) (out...) { /* here your mock goes */ }
func (o smth) m2 (in...) (out...) { return smthorig(in...) }

Types are aliased
interfaces should be used from smthorig intact

Beware of packages that have init and keep state.

Hope this helps.

--
Wojciech S. Czarnecki
<< ^oo^ >> OHIR-RIPE

[Nitish Saboo]

 I have a function in Go that I want to unit test but that function contains os.Hostname().Hence i thought of mocking os.Hostname.

Example:

func F(){

hostname, _ := os.Hostname()

}

I tried something like this:

var osHostname = os.Hostname

func TestF(t *testing.T) {
expected := "testing"
defer func() { osHostname = os.Hostname }()
osHostname = func()(string, error) { return "testing", nil }
actual := F()
assert.Equal(t,expected,actual)
}

But this doesn't work.Can someone please point me in the right direction?

Thanks,

Nitish

[Mhd Shulhan]

> On 20 Sep 2019, at 17.21, Nitish Saboo <nitish....@gmail.com> wrote:
>
> I have a function in Go that I want to unit test but that function contains os.Hostname().Hence i thought of mocking os.Hostname.
>
> Example:
>
> func F(){
> hostname, _ := os.Hostname()
> }
>
> I tried something like this:
>
> var osHostname = os.Hostname
>
> func TestF(t *testing.T) {
> expected := "testing"
> defer func() { osHostname = os.Hostname }()
> osHostname = func()(string, error) { return "testing", nil }
> actual := F()
> assert.Equal(t,expected,actual)
> }
>
> But this doesn't work.Can someone please point me in the right direction?

I have two notes about your code.
First, F() should have return values but its not.
Second, F() should use osHostname not os.Hostname.

The following pseudocode should works,

```
var osHostname = os.Hostname

func F() (string, error) {
return osHostname()
}

func TestF(t *testing.T) {
orgGetHostname := osHostname
osHostname = func() (string, error) {
return "testing", nil
}

defer func() {
osHostname = orgGetHostname
}()

exp := "testing"
got, _ := F()

if exp != got {
t.Fatalf("got %s, want %s", got, exp)
}
}
```

[Nitish Saboo]

Tried running you code, it's failing:

--- FAIL: TestF (0.00s)
    test.go:43: got ubuntu, want testing
FAIL

[Mhd Shulhan]

On Fri, 20 Sep 2019, 18:58 Nitish Saboo, <nitish....@gmail.com> wrote:

Tried running you code, it's failing:

--- FAIL: TestF (0.00s)
    test.go:43: got ubuntu, want testing
FAIL

If that's the case I think you should learn about programming in general first, before learning how to mock a function.  I believe there are many online tutorial in the web.

[Sam Whited]

On Thu, Aug 1, 2019, at 12:09, Nitish Saboo wrote:
> How can we mock a function in Go like os.Hostname() or os.Getwd()
> ? Any framework or Library that we can use for mocking the
> function calls ?

I don't like to think about "mocking" in Go, but I can't provide an
alternative term so maybe this is overly pedantic (or maybe one
isn't needed).

For something like this, I prefer to avoid global state like others have
suggested: eventually, you'll run a test in parallel, try to reuse that
state, etc and it will lead to race conditions. You may think you won't
now, but it's not a problem until it's a problem so you might as well
avoid it. Instead, I'd just pass in a hostname to use. If this isn't
something that makes sense in the public API, put it in an internal
function and test that:

// Foo is a public API that needs a hostname.
func Foo() {
hostname, err := os.Hostname()
// error handling

foo(hostname)
}

// test this
func foo(hostname string) {
// …
}

I also prefer to only test published methods using a _testing package,
and this flies in the face of that advice a bit but in a situation like
this I think it's by far the simplest solution that won't lead to
headaches later.

—Sam