Namespace: go.std.testing

v1.0

Contents

Summary

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)
}
}

# Benchmarks

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
https://golang.org/cmd/go/#hdr-Testing_flags.

A sample benchmark function looks like this:

func BenchmarkRandInt(b *testing.B) {
for i := 0; i < b.N; i++ {
rand.Int()
}
}

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()
b.ResetTimer()
for i := 0; i < b.N; i++ {
big.Len()
}
}

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() {
buf.Reset()
templ.Execute(&buf, "World")
}
})
}

A detailed specification of the benchmark results format is given
in https://golang.org/design/14313-benchmark-format.

There are standard tools for working with benchmark results at
https://golang.org/x/perf/cmd.
In particular, https://golang.org/x/perf/cmd/benchstat performs
statistically robust A/B comparisons.

# Examples

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() {
fmt.Println("hello")
// Output: hello
}

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

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

func ExamplePerm() {
for _, value := range Perm(5) {
fmt.Println(value)
}
// 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.

# Fuzzing

'go test' and the testing package support fuzzing, a testing technique where
a function is called with randomly generated inputs to find bugs not
anticipated by unit tests.

Functions of the form

func FuzzXxx(*testing.F)

are considered fuzz tests.

For example:

func FuzzHex(f *testing.F) {
for _, seed := range [][]byte{{}, {0}, {9}, {0xa}, {0xf}, {1, 2, 3, 4}} {
f.Add(seed)
}
f.Fuzz(func(t *testing.T, in []byte) {
enc := hex.EncodeToString(in)
out, err := hex.DecodeString(enc)
if err != nil {
t.Fatalf("%v: decode: %v", in, err)
}
if !bytes.Equal(in, out) {
t.Fatalf("%v: not equal after round trip: %v", in, out)
}
})
}

A fuzz test maintains a seed corpus, or a set of inputs which are run by
default, and can seed input generation. Seed inputs may be registered by
calling (*F).Add or by storing files in the directory testdata/fuzz/<Name>
(where <Name> is the name of the fuzz test) within the package containing
the fuzz test. Seed inputs are optional, but the fuzzing engine may find
bugs more efficiently when provided with a set of small seed inputs with good
code coverage. These seed inputs can also serve as regression tests for bugs
identified through fuzzing.

The function passed to (*F).Fuzz within the fuzz test is considered the fuzz
target. A fuzz target must accept a *T parameter, followed by one or more
parameters for random inputs. The types of arguments passed to (*F).Add must
be identical to the types of these parameters. The fuzz target may signal
that it's found a problem the same way tests do: by calling T.Fail (or any
method that calls it like T.Error or T.Fatal) or by panicking.

When fuzzing is enabled (by setting the -fuzz flag to a regular expression
that matches a specific fuzz test), the fuzz target is called with arguments
generated by repeatedly making random changes to the seed inputs. On
supported platforms, 'go test' compiles the test executable with fuzzing
coverage instrumentation. The fuzzing engine uses that instrumentation to
find and cache inputs that expand coverage, increasing the likelihood of
finding bugs. If the fuzz target fails for a given input, the fuzzing engine
writes the inputs that caused the failure to a file in the directory
testdata/fuzz/<Name> within the package directory. This file later serves as
a seed input. If the file can't be written at that location (for example,
because the directory is read-only), the fuzzing engine writes the file to
the fuzz cache directory within the build cache instead.

When fuzzing is disabled, the fuzz target is called with the seed inputs
registered with F.Add and seed inputs from testdata/fuzz/<Name>. In this
mode, the fuzz test acts much like a regular test, with subtests started
with F.Fuzz instead of T.Run.

See https://go.dev/doc/fuzz for documentation about fuzzing.

# Skipping

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.")
}
...
}

The Skip method of *T can be used in a fuzz target if the input is invalid,
but should not be considered a failing input. For example:

func FuzzJSONMarshaling(f *testing.F) {
f.Fuzz(func(t *testing.T, b []byte) {
var v interface{}
if err := json.Unmarshal(b, &v); err != nil {
t.Skip()
}
if _, err := json.Marshal(v); err != nil {
t.Error("Marshal: %v", err)
}
})
}

# 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, -bench, and -fuzz 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".
go test -fuzz FuzzFoo # Fuzz the target matching "FuzzFoo"

The -run argument can also be used to run a specific value in the seed
corpus, for debugging. For example:

go test -run=FuzzFoo/9ddb952d9814

The -fuzz and -run flags can both be set, in order to fuzz a target but
skip the execution of all other tests.

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) {
t.Parallel()
...
})
}
}

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>
}

# Main

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
os.Exit(m.Run())
}

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

Index

Legend

Constants

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.

Variables

Functions, Macros, and Special Forms

Types