Namespace: go.std.testing




Provides a low-level interface to the testing package.

Package testing provides support for automated testing of Go packages.
It is intended to be used in concert with the "go test" command, which automates
execution of any function of the form
func TestXxx(*testing.T)
where Xxx does not start with a lowercase letter. The function name
serves to identify the test routine.

Within these functions, use the Error, Fail or related methods to signal failure.

To write a new test suite, create a file whose name ends _test.go that
contains the TestXxx functions as described here. Put the file in the same
package as the one being tested. The file will be excluded from regular
package builds but will be included when the "go test" command is run.
For more detail, run "go help test" and "go help testflag".

A simple test function looks like this:

func TestAbs(t *testing.T) {
got := Abs(-1)
if got != 1 {
t.Errorf("Abs(-1) = %d; want 1", got)


Functions of the form
func BenchmarkXxx(*testing.B)
are considered benchmarks, and are executed by the "go test" command when
its -bench flag is provided. Benchmarks are run sequentially.

For a description of the testing flags, see

A sample benchmark function looks like this:
func BenchmarkRandInt(b *testing.B) {
for i := 0; i < b.N; i++ {

The benchmark function must run the target code b.N times.
During benchmark execution, b.N is adjusted until the benchmark function lasts
long enough to be timed reliably. The output
BenchmarkRandInt-8 68453040 17.8 ns/op
means that the loop ran 68453040 times at a speed of 17.8 ns per loop.

If a benchmark needs some expensive setup before running, the timer
may be reset:

func BenchmarkBigLen(b *testing.B) {
big := NewBig()
for i := 0; i < b.N; i++ {

If a benchmark needs to test performance in a parallel setting, it may use
the RunParallel helper function; such benchmarks are intended to be used with
the go test -cpu flag:

func BenchmarkTemplateParallel(b *testing.B) {
templ := template.Must(template.New("test").Parse("Hello, {{.}}!"))
b.RunParallel(func(pb *testing.PB) {
var buf bytes.Buffer
for pb.Next() {
templ.Execute(&buf, "World")


The package also runs and verifies example code. Example functions may
include a concluding line comment that begins with "Output:" and is compared with
the standard output of the function when the tests are run. (The comparison
ignores leading and trailing space.) These are examples of an example:

func ExampleHello() {
// Output: hello

func ExampleSalutations() {
fmt.Println("hello, and")
// Output:
// hello, and
// goodbye

The comment prefix "Unordered output:" is like "Output:", but matches any
line order:

func ExamplePerm() {
for _, value := range Perm(5) {
// Unordered output: 4
// 2
// 1
// 3
// 0

Example functions without output comments are compiled but not executed.

The naming convention to declare examples for the package, a function F, a type T and
method M on type T are:

func Example() { ... }
func ExampleF() { ... }
func ExampleT() { ... }
func ExampleT_M() { ... }

Multiple example functions for a package/type/function/method may be provided by
appending a distinct suffix to the name. The suffix must start with a
lower-case letter.

func Example_suffix() { ... }
func ExampleF_suffix() { ... }
func ExampleT_suffix() { ... }
func ExampleT_M_suffix() { ... }

The entire test file is presented as the example when it contains a single
example function, at least one other function, type, variable, or constant
declaration, and no test or benchmark functions.


Tests or benchmarks may be skipped at run time with a call to
the Skip method of *T or *B:

func TestTimeConsuming(t *testing.T) {
if testing.Short() {
t.Skip("skipping test in short mode.")

Subtests and Sub-benchmarks

The Run methods of T and B allow defining subtests and sub-benchmarks,
without having to define separate functions for each. This enables uses
like table-driven benchmarks and creating hierarchical tests.
It also provides a way to share common setup and tear-down code:

func TestFoo(t *testing.T) {
// <setup code>
t.Run("A=1", func(t *testing.T) { ... })
t.Run("A=2", func(t *testing.T) { ... })
t.Run("B=1", func(t *testing.T) { ... })
// <tear-down code>

Each subtest and sub-benchmark has a unique name: the combination of the name
of the top-level test and the sequence of names passed to Run, separated by
slashes, with an optional trailing sequence number for disambiguation.

The argument to the -run and -bench command-line flags is an unanchored regular
expression that matches the test's name. For tests with multiple slash-separated
elements, such as subtests, the argument is itself slash-separated, with
expressions matching each name element in turn. Because it is unanchored, an
empty expression matches any string.
For example, using "matching" to mean "whose name contains":

go test -run '' # Run all tests.
go test -run Foo # Run top-level tests matching "Foo", such as "TestFooBar".
go test -run Foo/A= # For top-level tests matching "Foo", run subtests matching "A=".
go test -run /A=1 # For all top-level tests, run subtests matching "A=1".

Subtests can also be used to control parallelism. A parent test will only
complete once all of its subtests complete. In this example, all tests are
run in parallel with each other, and only with each other, regardless of
other top-level tests that may be defined:

func TestGroupedParallel(t *testing.T) {
for _, tc := range tests {
tc := tc // capture range variable
t.Run(tc.Name, func(t *testing.T) {

The race detector kills the program if it exceeds 8128 concurrent goroutines,
so use care when running parallel tests with the -race flag set.

Run does not return until parallel subtests have completed, providing a way
to clean up after a group of parallel tests:

func TestTeardownParallel(t *testing.T) {
// This Run will not return until the parallel tests finish.
t.Run("group", func(t *testing.T) {
t.Run("Test1", parallelTest1)
t.Run("Test2", parallelTest2)
t.Run("Test3", parallelTest3)
// <tear-down code>


It is sometimes necessary for a test or benchmark program to do extra setup or teardown
before or after it executes. It is also sometimes necessary to control
which code runs on the main thread. To support these and other cases,
if a test file contains a function:

func TestMain(m *testing.M)

then the generated test will call TestMain(m) instead of running the tests or benchmarks
directly. TestMain runs in the main goroutine and can do whatever setup
and teardown is necessary around a call to m.Run. m.Run will return an exit
code that may be passed to os.Exit. If TestMain returns, the test wrapper
will pass the result of m.Run to os.Exit itself.

When TestMain is called, flag.Parse has not been run. If TestMain depends on
command-line flags, including those of the testing package, it should call
flag.Parse explicitly. Command line flags are always parsed by the time test
or benchmark functions run.

A simple implementation of TestMain is:

func TestMain(m *testing.M) {
// call flag.Parse() here if TestMain uses flags

TestMain is a low-level primitive and should not be necessary for casual
testing needs, where ordinary test functions suffice.




Constants are variables with :const true in their metadata. Joker currently does not recognize them as special; as such, it allows redefining them or their values.


Functions, Macros, and Special Forms