Package testing

import "testing" Overview Index Examples Subdirectories

Overview ▹ Overview ▾ 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") } }) } 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. 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.") } ... } 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) { t.Parallel() ... }) } } The race detector kills the program if it exceeds 8192 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> } Main It is sometimes necessary for a test program to do extra setup or teardown before or after testing. It is also sometimes necessary for a test 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 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()) }

func AllocsPerRun(runs int, f func()) (avg float64)

AllocsPerRun returns the average number of allocations during calls to f. Although the return value has type float64, it will always be an integral value.

To compute the number of allocations, the function will first be run once as a warm-up. The average number of allocations over the specified number of runs will then be measured and returned.

AllocsPerRun sets GOMAXPROCS to 1 during its measurement and will restore it before returning.

func CoverMode() string

CoverMode reports what the test coverage mode is set to. The values are "set", "count", or "atomic". The return value will be empty if test coverage is not enabled.

func Coverage() float64

Coverage reports the current code coverage as a fraction in the range [0, 1]. If coverage is not enabled, Coverage returns 0.

When running a large set of sequential test cases, checking Coverage after each one can be useful for identifying which test cases exercise new code paths. It is not a replacement for the reports generated by 'go test -cover' and 'go tool cover'.

func Init()

Init registers testing flags. These flags are automatically registered by the "go test" command before running test functions, so Init is only needed when calling functions such as Benchmark without using "go test".

Init has no effect if it was already called.

func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample)

Main is an internal function, part of the implementation of the "go test" command. It was exported because it is cross-package and predates "internal" packages. It is no longer used by "go test" but preserved, as much as possible, for other systems that simulate "go test" using Main, but Main sometimes cannot be updated as new functionality is added to the testing package. Systems simulating "go test" should be updated to use MainStart.

func RegisterCover(c Cover)

RegisterCover records the coverage data accumulators for the tests. NOTE: This function is internal to the testing infrastructure and may change. It is not covered (yet) by the Go 1 compatibility guidelines.

func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark)

RunBenchmarks is an internal function but exported because it is cross-package; it is part of the implementation of the "go test" command.

func RunExamples(matchString func(pat, str string) (bool, error), examples []InternalExample) (ok bool)

RunExamples is an internal function but exported because it is cross-package; it is part of the implementation of the "go test" command.

func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool)

RunTests is an internal function but exported because it is cross-package; it is part of the implementation of the "go test" command.

func Short() bool

Short reports whether the -test.short flag is set.

func Verbose() bool

Verbose reports whether the -test.v flag is set.

B is a type passed to Benchmark functions to manage benchmark timing and to specify the number of iterations to run.

A benchmark ends when its Benchmark function returns or calls any of the methods FailNow, Fatal, Fatalf, SkipNow, Skip, or Skipf. Those methods must be called only from the goroutine running the Benchmark function. The other reporting methods, such as the variations of Log and Error, may be called simultaneously from multiple goroutines.

Like in tests, benchmark logs are accumulated during execution and dumped to standard output when done. Unlike in tests, benchmark logs are always printed, so as not to hide output whose existence may be affecting benchmark results.

type B struct { N int }

func (c *B) Cleanup(f func())

Cleanup registers a function to be called when the test and all its subtests complete. Cleanup functions will be called in last added, first called order.

func (c *B) Error(args ...interface{})

Error is equivalent to Log followed by Fail.

func (c *B) Errorf(format string, args ...interface{})

Errorf is equivalent to Logf followed by Fail.

func (c *B) Fail()

Fail marks the function as having failed but continues execution.

func (c *B) FailNow()

FailNow marks the function as having failed and stops its execution by calling runtime.Goexit (which then runs all deferred calls in the current goroutine). Execution will continue at the next test or benchmark. FailNow must be called from the goroutine running the test or benchmark function, not from other goroutines created during the test. Calling FailNow does not stop those other goroutines.

func (c *B) Failed() bool

Failed reports whether the function has failed.

func (c *B) Fatal(args ...interface{})

Fatal is equivalent to Log followed by FailNow.

func (c *B) Fatalf(format string, args ...interface{})

Fatalf is equivalent to Logf followed by FailNow.

func (c *B) Helper()

Helper marks the calling function as a test helper function. When printing file and line information, that function will be skipped. Helper may be called simultaneously from multiple goroutines.

func (c *B) Log(args ...interface{})

Log formats its arguments using default formatting, analogous to Println, and records the text in the error log. For tests, the text will be printed only if the test fails or the -test.v flag is set. For benchmarks, the text is always printed to avoid having performance depend on the value of the -test.v flag.

func (c *B) Logf(format string, args ...interface{})

Logf formats its arguments according to the format, analogous to Printf, and records the text in the error log. A final newline is added if not provided. For tests, the text will be printed only if the test fails or the -test.v flag is set. For benchmarks, the text is always printed to avoid having performance depend on the value of the -test.v flag.

func (c *B) Name() string

Name returns the name of the running test or benchmark.

func (b *B) ReportAllocs()

ReportAllocs enables malloc statistics for this benchmark. It is equivalent to setting -test.benchmem, but it only affects the benchmark function that calls ReportAllocs.

func (b *B) ReportMetric(n float64, unit string)

ReportMetric adds "n unit" to the reported benchmark results. If the metric is per-iteration, the caller should divide by b.N, and by convention units should end in "/op". ReportMetric overrides any previously reported value for the same unit. ReportMetric panics if unit is the empty string or if unit contains any whitespace. If unit is a unit normally reported by the benchmark framework itself (such as "allocs/op"), ReportMetric will override that metric. Setting "ns/op" to 0 will suppress that built-in metric.

▹ Example ▾ Example package main import ( "sort" "testing" ) func main() { // This reports a custom benchmark metric relevant to a // specific algorithm (in this case, sorting). testing.Benchmark(func(b *testing.B) { var compares int64 for i := 0; i < b.N; i++ { s := []int{5, 4, 3, 2, 1} sort.Slice(s, func(i, j int) bool { compares++ return s[i] < s[j] }) } // This metric is per-operation, so divide by b.N and // report it as a "/op" unit. b.ReportMetric(float64(compares)/float64(b.N), "compares/op") }) } Run Format

func (b *B) ResetTimer()

ResetTimer zeroes the elapsed benchmark time and memory allocation counters and deletes user-reported metrics. It does not affect whether the timer is running.

func (b *B) Run(name string, f func(b *B)) bool

Run benchmarks f as a subbenchmark with the given name. It reports whether there were any failures.

A subbenchmark is like any other benchmark. A benchmark that calls Run at least once will not be measured itself and will be called once with N=1.

func (b *B) RunParallel(body func(*PB))

RunParallel runs a benchmark in parallel. It creates multiple goroutines and distributes b.N iterations among them. The number of goroutines defaults to GOMAXPROCS. To increase parallelism for non-CPU-bound benchmarks, call SetParallelism before RunParallel. RunParallel is usually used with the go test -cpu flag.

The body function will be run in each goroutine. It should set up any goroutine-local state and then iterate until pb.Next returns false. It should not use the StartTimer, StopTimer, or ResetTimer functions, because they have global effect. It should also not call Run.

▹ Example ▾ Example package main import ( "bytes" "testing" "text/template" ) func main() { // Parallel benchmark for text/template.Template.Execute on a single object. testing.Benchmark(func(b *testing.B) { templ := template.Must(template.New("test").Parse("Hello, {{.}}!")) // RunParallel will create GOMAXPROCS goroutines // and distribute work among them. b.RunParallel(func(pb *testing.PB) { // Each goroutine has its own bytes.Buffer. var buf bytes.Buffer for pb.Next() { // The loop body is executed b.N times total across all goroutines. buf.Reset() templ.Execute(&buf, "World") } }) }) } Run Format

func (b *B) SetBytes(n int64)

SetBytes records the number of bytes processed in a single operation. If this is called, the benchmark will report ns/op and MB/s.

func (b *B) SetParallelism(p int)

SetParallelism sets the number of goroutines used by RunParallel to p*GOMAXPROCS. There is usually no need to call SetParallelism for CPU-bound benchmarks. If p is less than 1, this call will have no effect.

func (c *B) Skip(args ...interface{})

Skip is equivalent to Log followed by SkipNow.

func (c *B) SkipNow()

SkipNow marks the test as having been skipped and stops its execution by calling runtime.Goexit. If a test fails (see Error, Errorf, Fail) and is then skipped, it is still considered to have failed. Execution will continue at the next test or benchmark. See also FailNow. SkipNow must be called from the goroutine running the test, not from other goroutines created during the test. Calling SkipNow does not stop those other goroutines.

func (c *B) Skipf(format string, args ...interface{})

Skipf is equivalent to Logf followed by SkipNow.

func (c *B) Skipped() bool

Skipped reports whether the test was skipped.

func (b *B) StartTimer()

StartTimer starts timing a test. This function is called automatically before a benchmark starts, but it can also be used to resume timing after a call to StopTimer.

func (b *B) StopTimer()

StopTimer stops timing a test. This can be used to pause the timer while performing complex initialization that you don't want to measure.

func (c *B) TempDir() string

TempDir returns a temporary directory for the test to use. The directory is automatically removed by Cleanup when the test and all its subtests complete. Each subsequent call to t.TempDir returns a unique directory; if the directory creation fails, TempDir terminates the test by calling Fatal.

BenchmarkResult contains the results of a benchmark run.

type BenchmarkResult struct { N int T time.Duration Bytes int64 MemAllocs uint64 MemBytes uint64 Extra map[string]float64 }

func Benchmark(f func(b *B)) BenchmarkResult

Benchmark benchmarks a single function. It is useful for creating custom benchmarks that do not use the "go test" command.

If f depends on testing flags, then Init must be used to register those flags before calling Benchmark and before calling flag.Parse.

If f calls Run, the result will be an estimate of running all its subbenchmarks that don't call Run in sequence in a single benchmark.

func (r BenchmarkResult) AllocedBytesPerOp() int64

AllocedBytesPerOp returns the "B/op" metric, which is calculated as r.MemBytes / r.N.

func (r BenchmarkResult) AllocsPerOp() int64

AllocsPerOp returns the "allocs/op" metric, which is calculated as r.MemAllocs / r.N.

func (r BenchmarkResult) MemString() string

MemString returns r.AllocedBytesPerOp and r.AllocsPerOp in the same format as 'go test'.

func (r BenchmarkResult) NsPerOp() int64

NsPerOp returns the "ns/op" metric.

func (r BenchmarkResult) String() string

String returns a summary of the benchmark results. It follows the benchmark result line format from https://golang.org/design/14313-benchmark-format, not including the benchmark name. Extra metrics override built-in metrics of the same name. String does not include allocs/op or B/op, since those are reported by MemString.

Cover records information about test coverage checking. NOTE: This struct is internal to the testing infrastructure and may change. It is not covered (yet) by the Go 1 compatibility guidelines.

type Cover struct { Mode string Counters map[string][]uint32 Blocks map[string][]CoverBlock CoveredPackages string }

CoverBlock records the coverage data for a single basic block. The fields are 1-indexed, as in an editor: The opening line of the file is number 1, for example. Columns are measured in bytes. NOTE: This struct is internal to the testing infrastructure and may change. It is not covered (yet) by the Go 1 compatibility guidelines.

type CoverBlock struct { Line0 uint32 Col0 uint16 Line1 uint32 Col1 uint16 Stmts uint16 }

InternalBenchmark is an internal type but exported because it is cross-package; it is part of the implementation of the "go test" command.

type InternalBenchmark struct { Name string F func(b *B) }

type InternalExample struct { Name string F func() Output string Unordered bool }

InternalTest is an internal type but exported because it is cross-package; it is part of the implementation of the "go test" command.

type InternalTest struct { Name string F func(*T) }

M is a type passed to a TestMain function to run the actual tests.

type M struct { }

func MainStart(deps testDeps, tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample) *M

MainStart is meant for use by tests generated by 'go test'. It is not meant to be called directly and is not subject to the Go 1 compatibility document. It may change signature from release to release.

func (m *M) Run() (code int)

Run runs the tests. It returns an exit code to pass to os.Exit.

A PB is used by RunParallel for running parallel benchmarks.

type PB struct { }

func (pb *PB) Next() bool

Next reports whether there are more iterations to execute.

T is a type passed to Test functions to manage test state and support formatted test logs.

A test ends when its Test function returns or calls any of the methods FailNow, Fatal, Fatalf, SkipNow, Skip, or Skipf. Those methods, as well as the Parallel method, must be called only from the goroutine running the Test function.

The other reporting methods, such as the variations of Log and Error, may be called simultaneously from multiple goroutines.

type T struct { }

func (c *T) Cleanup(f func())

Cleanup registers a function to be called when the test and all its subtests complete. Cleanup functions will be called in last added, first called order.

func (t *T) Deadline() (deadline time.Time, ok bool)

Deadline reports the time at which the test binary will have exceeded the timeout specified by the -timeout flag.

The ok result is false if the -timeout flag indicates “no timeout” (0).

func (c *T) Error(args ...interface{})

Error is equivalent to Log followed by Fail.

func (c *T) Errorf(format string, args ...interface{})

Errorf is equivalent to Logf followed by Fail.

func (c *T) Fail()

Fail marks the function as having failed but continues execution.

func (c *T) FailNow()

FailNow marks the function as having failed and stops its execution by calling runtime.Goexit (which then runs all deferred calls in the current goroutine). Execution will continue at the next test or benchmark. FailNow must be called from the goroutine running the test or benchmark function, not from other goroutines created during the test. Calling FailNow does not stop those other goroutines.

func (c *T) Failed() bool

Failed reports whether the function has failed.

func (c *T) Fatal(args ...interface{})

Fatal is equivalent to Log followed by FailNow.

func (c *T) Fatalf(format string, args ...interface{})

Fatalf is equivalent to Logf followed by FailNow.

func (c *T) Helper()

Helper marks the calling function as a test helper function. When printing file and line information, that function will be skipped. Helper may be called simultaneously from multiple goroutines.

func (c *T) Log(args ...interface{})

Log formats its arguments using default formatting, analogous to Println, and records the text in the error log. For tests, the text will be printed only if the test fails or the -test.v flag is set. For benchmarks, the text is always printed to avoid having performance depend on the value of the -test.v flag.

func (c *T) Logf(format string, args ...interface{})

Logf formats its arguments according to the format, analogous to Printf, and records the text in the error log. A final newline is added if not provided. For tests, the text will be printed only if the test fails or the -test.v flag is set. For benchmarks, the text is always printed to avoid having performance depend on the value of the -test.v flag.

func (c *T) Name() string

Name returns the name of the running test or benchmark.

func (t *T) Parallel()

Parallel signals that this test is to be run in parallel with (and only with) other parallel tests. When a test is run multiple times due to use of -test.count or -test.cpu, multiple instances of a single test never run in parallel with each other.

func (t *T) Run(name string, f func(t *T)) bool

Run runs f as a subtest of t called name. It runs f in a separate goroutine and blocks until f returns or calls t.Parallel to become a parallel test. Run reports whether f succeeded (or at least did not fail before calling t.Parallel).

Run may be called simultaneously from multiple goroutines, but all such calls must return before the outer test function for t returns.

func (c *T) Skip(args ...interface{})

Skip is equivalent to Log followed by SkipNow.

func (c *T) SkipNow()

SkipNow marks the test as having been skipped and stops its execution by calling runtime.Goexit. If a test fails (see Error, Errorf, Fail) and is then skipped, it is still considered to have failed. Execution will continue at the next test or benchmark. See also FailNow. SkipNow must be called from the goroutine running the test, not from other goroutines created during the test. Calling SkipNow does not stop those other goroutines.

func (c *T) Skipf(format string, args ...interface{})

Skipf is equivalent to Logf followed by SkipNow.

func (c *T) Skipped() bool

Skipped reports whether the test was skipped.

func (c *T) TempDir() string

TempDir returns a temporary directory for the test to use. The directory is automatically removed by Cleanup when the test and all its subtests complete. Each subsequent call to t.TempDir returns a unique directory; if the directory creation fails, TempDir terminates the test by calling Fatal.

TB is the interface common to T and B.

type TB interface { Cleanup(func()) Error(args ...interface{}) Errorf(format string, args ...interface{}) Fail() FailNow() Failed() bool Fatal(args ...interface{}) Fatalf(format string, args ...interface{}) Helper() Log(args ...interface{}) Logf(format string, args ...interface{}) Name() string Skip(args ...interface{}) SkipNow() Skipf(format string, args ...interface{}) Skipped() bool TempDir() string }

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