Golang testing — gotest.tools assertions

Published on Aug 16, 2018 by Vincent Demeester.


Let’s take a closer look at gotest.tools assertions packages. This is mainly about assert, assert/cmp and assert/opt.

Package assert provides assertions for comparing expected values to actual values. When assertion fails a helpful error message is printed.

There is two main functions (Assert and Check) and some helpers (like NilError, …). They all take a *testing.T as a first argument, pretty common across testing Go libraries. Let’s dive into those !

Assert and Check

Both those functions accept a Comparison (we’ll check what it is later on) and fail the test when that comparison fails. The one difference is that Assert will end the test execution at immediately whereas Check will fail the test and proceed with the rest of the test case. This is similar to FailNow and Fail from the standard library testing. Both have their use cases.

We’ll Use Assert for the rest of the section but any example here would work with Check too. When we said Comparison above, it’s mainly the BoolOrComparison interface — it can either be a boolean expression, or a cmp.Comparison type. Assert and Check code will be smart enough to detect which one it is.

assert.Assert(t, ok)
assert.Assert(t, err != nil)
assert.Assert(t, foo.IsBar())

So far not anything extra-ordinary. Let’s first look at some more helper functions in the assert package and quickly dive a bit deeper with Comparison.

More assert helpers

The additional helper functions are the following

  • Equal uses the == operator to assert two values are equal.
  • DeepEqual uses google/go-cmp to assert two values are equal (it’s close to reflect.DeepEqual but not quite). We’ll detail a bit more the options part of this function with cmp.DeepEqual.
  • Error fails if the error is nil or the error message is not the expected one.
  • ErrorContains fails if the error is nil or the error message does not contain the expected substring.
  • ErrorType fails if the error is nil or the error type is not the expected type.
  • NilError fails if the error is not nil.

All those helper functions have a equivalent function in the cmp package that returns a Comparison. I, personally, prefer to use assert.Check or assert.Assert in combination with cmp.Comparison as it allows me to write all my assertions the same way, with built-ins comparison or with my own — i.e. assert.Assert(t, is.Equal(…), "message" or assert.Assert(t, stackIsUp(c, time…), "another message").


This is where it get really interesting, gotest.tools tries to make it as easy as possible for you to create appropriate comparison — making you test readable as much as possible.

Let’s look a bit at the cmp.Comparison type.

type Comparison func() Result

It’s just a function that returns a cmp.Result, so let’s look at cmp.Result definition.

type Result interface {
        Success() bool

Result is an interface, thus any struct that provide a function Success that returns a bool can be used as a comparison result, making it really easy to use in your code. There is also existing type of result to make it even quicker to write your own comparison.

  • ResultSuccess is a constant which is returned to indicate success.
  • ResultFailure and ResultFailureTemplate return a failed Result with a failure message.
  • ResultFromError returns ResultSuccess if err is nil. Otherwise ResultFailure is returned with the error message as the failure message. It works a bit like the errors.Wrap function of the github.com/pkgs/errors package.

The cmp package comes with a few defined comparison that, we think, should cover a high number of use-cases. Let’s look at them.

Equality with Equal and DeepEqual

Equal uses the == operator to assert two values are equal and fails the test if they are not equal.

If the comparison fails Equal will use the variable names for x and y as part of the failure message to identify the actual and expected values.

If either x or y are a multi-line string the failure message will include a unified diff of the two values. If the values only differ by whitespace the unified diff will be augmented by replacing whitespace characters with visible characters to identify the whitespace difference.

On the other hand…

DeepEqual uses google/go-cmp (http://bit.do/go-cmp) to assert two values are equal and fails the test if they are not equal.

Package https://godoc.org/gotest.tools/assert/opt provides some additional commonly used Options.

Using one or the other is as simple as : if you wrote your if with == then use Equal, otherwise use DeepEqual. DeepEqual (and usually reflect.DeepEqual) is used when you want to compare anything more complex than primitive types. One advantage of using cmp.DeepEqual over reflect.DeepEqual (in an if), is that you get a well crafted message that shows the diff between the expected and the actual structs compared – and you can pass options to it.

assert.Assert(t, cmp.DeepEqual([]string{"a", "b"}, []string{"b", "a"}))
// Will print something like
// --- result
// +++ exp
// {[]string}[0]:
//         -: "a"
//         +: "b"
// {[]string}[1]:
//         -: "b"
//         +: "a"
foo := &someType(a: "with", b: "value")
bar := &someType(a: "with", b: "value")
// the following will succeed as foo and bar are _DeepEqual_
assert.Assert(t, cmp.DeepEqual(foo, bar))

When using DeepEqual, you may end up with really weird behavior(s). You may want to ignore some fields, or consider nil slice or map the same as empty ones ; or more common, your struct contains some unexported fields that you cannot use when comparing (as they are not exported 😓). In those case, you can use go-cmp options.

Some existing one are :

  • EquateEmpty returns a Comparer option that determines all maps and slices with a length of zero to be equal, regardless of whether they are nil.
  • IgnoreFields returns an Option that ignores exported fields of the given names on a single struct type. The struct type is specified by passing in a value of that type.
  • IgnoreUnexported returns an Option that only ignores the immediate unexported fields of a struct, including anonymous fields of unexported types.
  • SortSlices returns a Transformer option that sorts all []V
  • … and more 👼

gotest.tools also defines some and you can define yours ! As an example, gotest.tools defines TimeWithThreshold and DurationWithThreshold that allows to not fails if the time (or duration) is not exactly the same but in the specified threshold we specified. Here is the code for DurationWithThreshold for inspiration.

// DurationWithThreshold returns a gocmp.Comparer for comparing time.Duration. The
// Comparer returns true if the difference between the two Duration values is
// within the threshold and neither value is zero.
func DurationWithThreshold(threshold time.Duration) gocmp.Option {
        return gocmp.Comparer(cmpDuration(threshold))

func cmpDuration(threshold time.Duration) func(x, y time.Duration) bool {
        return func(x, y time.Duration) bool {
                if x == 0 || y == 0 {
                        return false
                delta := x - y
                return delta <= threshold && delta >= -threshold

Another good example for those options is when you want to skip some field. In docker/docker we want to be able to easily check for equality between two service specs, but those might have different CreatedAt and UpdatedAt values that we usually don’t care about – what we want is to make sure it happens in the past 20 seconds. You can easily define an option for that.

func cmpServiceOpts() cmp.Option {
        const threshold = 20 * time.Second

        // Apply withinThreshold only for the following fields
        metaTimeFields := func(path cmp.Path)bool {
                switch path.String() {
                case "Meta.CreatedAt", "Meta.UpdatedAt":
                        return true
                return false
        // have a 20s threshold for the time value that will be passed
        withinThreshold := cmp.Comparer(func(x, y time.Time) bool {
                delta := x.Sub(y)
                return delta < threshold && delta > -threshold

        return cmp.FilterPath(metaTimeFields, withinThreshold)

I recommend you look at the gotest.tools/assert/opt documentation to see which one are defined and how to use them.

Errors with Error, ErrorContains and ErrorType

Checking for errors is very common in Go, having Comparison function for it was a requirement.

  • Error fails if the error is nil or the error message is not the expected one.
  • ErrorContains fails if the error is nil or the error message does not contain the expected substring.
  • ErrorType fails if the error is nil or the error type is not the expected type.

Let’s first look at the most used : Error and ErrorContains.

var err error
// will fail with : expected an error, got nil
assert.Check(t, cmp.Error(err, "message in a bottle"))
err = errors.Wrap(errors.New("other"), "wrapped")
// will fail with : expected error "other", got "wrapped: other"
assert.Check(t, cmp.Error(err, "other"))
// will succeed
assert.Check(t, cmp.ErrorContains(err, "other"))

As you can see ErrorContains is especially useful when working with wrapped errors. Now let’s look at ErrorType.

var err error
// will fail with : error is nil, not StubError
assert.Check(t, cmp.ErrorType(err, StubError{}))

err := StubError{"foo"}
// will succeed
assert.Check(t, cmp.ErrorType(err, StubError{}))

// Note that it also work with a function returning an error
func foo() error {}
assert.Check(t, cmp.ErrorType(foo, StubError{}))

Bonus with Panics

Sometimes, a code is supposed to panic, see Effective Go (#Panic) for more information. And thus, you may want to make sure you’re code panics in such cases. It’s always a bit tricky to test a code that panic as you have to use a deferred function to recover the panic — but then if the panic doesn’t happen how do you fail the test ?

This is where Panics comes handy.

func foo(shouldPanic bool) {
        if shouldPanic {
        // don't worry, be happy
// will fail with : did not panic
assert.Check(t, cmp.Panics(foo(false)))
// will succeed
assert.Check(t, cmp.Panics(foo(true)))

Miscellaneous with Contains, Len and Nil

Those last three built-in Comparison are pretty straightforward.

  • Contains succeeds if item is in collection. Collection may be a string, map, slice, or array.

    If collection is a string, item must also be a string, and is compared using strings.Contains(). If collection is a Map, contains will succeed if item is a key in the map. If collection is a slice or array, item is compared to each item in the sequence using =reflect.DeepEqual()=.

  • Len succeeds if the sequence has the expected length.
  • Nil succeeds if obj is a nil interface, pointer, or function.
// Contains works on string, map, slice or arrays
assert.Check(t, cmp.Contains("foobar", "foo"))
assert.Check(t, cmp.Contains([]string{"a", "b", "c"}, "b"))
assert.Check(t, cmp.Contains(map[string]int{"a": 1, "b": 2, "c": 4}, "b"))

// Len also works on string, map, slice or arrays
assert.Check(t, cmp.Len("foobar", 6))
assert.Check(t, cmp.Len([]string{"a", "b", "c"}, 3))
assert.Check(t, cmp.Len(map[string]int{"a": 1, "b": 2, "c": 4}, 3))

// Nil
var foo *MyStruc
assert.Check(t, cmp.Nil(foo))
assert.Check(t, cmp.Nil(bar()))

But let’s not waste more time and let’s see how to write our own Comparison !

Write your own Comparison

One of the main aspect of gotest.tools/assert is to make it easy for developer to write as less boilerplate code as possible while writing tests. Writing your own Comparison allows you to write a well named function that will be easy to read and that can be re-used across your tests.

Let’s look back at the cmp.Comparison and cmp.Result types.

type Comparison func() Result

type Result interface {
        Success() bool

A Comparison for assert.Check or assert.Check is a function that return a Result, it’s pretty straightforward to implement, especially with cmp.ResultSuccess and cmp.ResultFailure(…) (as seen previously).

func regexPattern(value string, pattern string) cmp.Comparison {
        return func() cmp.Result {
                re := regexp.MustCompile(pattern)
                if re.MatchString(value) {
                        return cmp.ResultSuccess
                return cmp.ResultFailure(
                        fmt.Sprintf("%q did not match pattern %q", value, pattern))

// To use it
assert.Check(t, regexPattern("12345.34", `\d+.\d\d`))

As you can see, it’s pretty easy to implement, and you can do quite a lot in there easily. If a function call returns an error inside of your Comparison function, you can use cmp.ResultFromError for example. Having something like assert.Check(t, isMyServerUp(":8080")) is way more readable than a 30-line of code to check it.


… and that’s a wrap. We only looked at the assert package of gotest.tools so far, but it’s already quite a bit to process.

We’ve seen :

  • the main functions provided by this package : assert.Assert and assert.Check
  • some helper functions like assert.NilError, …
  • the assert/cmp, and assert/opt sub-package that allows you to write more custom Comparison

Next time, we’ll look at the skip package, that is a really simple wrapper on top of testing.Skip function.