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Package testing

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 can be any alphanumeric string (but the first letter must not be in [a-z]) and 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”.

Tests and benchmarks may be skipped if not applicable with a call to the Skip method of *T and *B: 

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

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-Description_of_testing_flags.

A sample benchmark function looks like this: 

func BenchmarkHello(b *testing.B) {
    for i := 0; i < b.N; i++ {
        fmt.Sprintf("hello")
    }
}

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 

BenchmarkHello    10000000    282 ns/op

means that the loop ran 10000000 times at a speed of 282 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
}

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.

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()
            ...
        })
    }
}

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. It should then call os.Exit with the result of m.Run. 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.

A simple implementation of TestMain is: 

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

Index ▾

func AllocsPerRun(runs int, f func()) (avg float64)
func CoverMode() string
func Coverage() float64
func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample)
func RegisterCover(c Cover)
func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark)
func RunExamples(matchString func(pat, str string) (bool, error), examples []InternalExample) (ok bool)
func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool)
func Short() bool
func Verbose() bool
type B
    func (c *B) Error(args ...interface{})
    func (c *B) Errorf(format string, args ...interface{})
    func (c *B) Fail()
    func (c *B) FailNow()
    func (c *B) Failed() bool
    func (c *B) Fatal(args ...interface{})
    func (c *B) Fatalf(format string, args ...interface{})
    func (c *B) Log(args ...interface{})
    func (c *B) Logf(format string, args ...interface{})
    func (c *B) Name() string
    func (b *B) ReportAllocs()
    func (b *B) ResetTimer()
    func (b *B) Run(name string, f func(b *B)) bool
    func (b *B) RunParallel(body func(*PB))
    func (b *B) SetBytes(n int64)
    func (b *B) SetParallelism(p int)
    func (c *B) Skip(args ...interface{})
    func (c *B) SkipNow()
    func (c *B) Skipf(format string, args ...interface{})
    func (c *B) Skipped() bool
    func (b *B) StartTimer()
    func (b *B) StopTimer()
type BenchmarkResult
    func Benchmark(f func(b *B)) BenchmarkResult
    func (r BenchmarkResult) AllocedBytesPerOp() int64
    func (r BenchmarkResult) AllocsPerOp() int64
    func (r BenchmarkResult) MemString() string
    func (r BenchmarkResult) NsPerOp() int64
    func (r BenchmarkResult) String() string
type Cover
type CoverBlock
type InternalBenchmark
type InternalExample
type InternalTest
type M
    func MainStart(deps testDeps, tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample) *M
    func (m *M) Run() int
type PB
    func (pb *PB) Next() bool
type T
    func (c *T) Error(args ...interface{})
    func (c *T) Errorf(format string, args ...interface{})
    func (c *T) Fail()
    func (c *T) FailNow()
    func (c *T) Failed() bool
    func (c *T) Fatal(args ...interface{})
    func (c *T) Fatalf(format string, args ...interface{})
    func (c *T) Log(args ...interface{})
    func (c *T) Logf(format string, args ...interface{})
    func (c *T) Name() string
    func (t *T) Parallel()
    func (t *T) Run(name string, f func(t *T)) bool
    func (c *T) Skip(args ...interface{})
    func (c *T) SkipNow()
    func (c *T) Skipf(format string, args ...interface{})
    func (c *T) Skipped() bool
type TB

Examples

B.RunParallel

Package files

allocs.go benchmark.go cover.go example.go match.go testing.go

func AllocsPerRun 

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 

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 

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 Main 

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 

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 

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

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

func RunExamples 

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

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

func RunTests 

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

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

func Short 

func Short() bool

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

func Verbose 

func Verbose() bool

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

type B

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 error 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
    // contains filtered or unexported fields
}

func (*B) Error 

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

Error is equivalent to Log followed by Fail.

func (*B) Errorf 

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

Errorf is equivalent to Logf followed by Fail.

func (*B) Fail 

func (c *B) Fail()

Fail marks the function as having failed but continues execution.

func (*B) FailNow 

func (c *B) FailNow()

FailNow marks the function as having failed and stops its execution. 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 (*B) Failed 

func (c *B) Failed() bool

Failed reports whether the function has failed.

func (*B) Fatal 

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

Fatal is equivalent to Log followed by FailNow.

func (*B) Fatalf 

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

Fatalf is equivalent to Logf followed by FailNow.

func (*B) Log 

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 (*B) Logf 

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 (*B) Name 

func (c *B) Name() string

Name returns the name of the running test or benchmark.

func (*B) ReportAllocs 

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) ResetTimer 

func (b *B) ResetTimer()

ResetTimer zeros the elapsed benchmark time and memory allocation counters. It does not affect whether the timer is running.

func (*B) Run 

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.

Run may be called simultaneously from multiple goroutines, but all such calls must happen before the outer benchmark function for b returns.

func (*B) RunParallel 

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

Code:

// 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")
        }
    })
})

func (*B) SetBytes 

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) SetParallelism 

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 (*B) Skip 

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

Skip is equivalent to Log followed by SkipNow.

func (*B) SkipNow 

func (c *B) SkipNow()

SkipNow marks the test as having been skipped and stops its execution. 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 (*B) Skipf 

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

Skipf is equivalent to Logf followed by SkipNow.

func (*B) Skipped 

func (c *B) Skipped() bool

Skipped reports whether the test was skipped.

func (*B) StartTimer 

func (b *B) StartTimer()

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

func (*B) 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.

type BenchmarkResult

The results of a benchmark run. 

type BenchmarkResult struct {
    N         int           // The number of iterations.
    T         time.Duration // The total time taken.
    Bytes     int64         // Bytes processed in one iteration.
    MemAllocs uint64        // The total number of memory allocations.
    MemBytes  uint64        // The total number of bytes allocated.
}

func Benchmark 

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

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

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 (BenchmarkResult) AllocedBytesPerOp 

func (r BenchmarkResult) AllocedBytesPerOp() int64

func (BenchmarkResult) AllocsPerOp 

func (r BenchmarkResult) AllocsPerOp() int64

func (BenchmarkResult) MemString 

func (r BenchmarkResult) MemString() string

func (BenchmarkResult) NsPerOp 

func (r BenchmarkResult) NsPerOp() int64

func (BenchmarkResult) String 

func (r BenchmarkResult) String() string

type Cover

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
}

type CoverBlock

CoverBlock records the coverage data for a single basic block. 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
}

type InternalBenchmark

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

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

type InternalExample 

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

type InternalTest

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

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

type M

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

type M struct {
    // contains filtered or unexported fields
}

func MainStart 

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) Run 

func (m *M) Run() int

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

type PB

A PB is used by RunParallel for running parallel benchmarks. 

type PB struct {
    // contains filtered or unexported fields
}

func (*PB) Next 

func (pb *PB) Next() bool

Next reports whether there are more iterations to execute.

type T

T is a type passed to Test functions to manage test state and support formatted test logs. Logs are accumulated during execution and dumped to standard output when done.

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 {
    // contains filtered or unexported fields
}

func (*T) Error 

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

Error is equivalent to Log followed by Fail.

func (*T) Errorf 

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

Errorf is equivalent to Logf followed by Fail.

func (*T) Fail 

func (c *T) Fail()

Fail marks the function as having failed but continues execution.

func (*T) FailNow 

func (c *T) FailNow()

FailNow marks the function as having failed and stops its execution. 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 (*T) Failed 

func (c *T) Failed() bool

Failed reports whether the function has failed.

func (*T) Fatal 

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

Fatal is equivalent to Log followed by FailNow.

func (*T) Fatalf 

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

Fatalf is equivalent to Logf followed by FailNow.

func (*T) Log 

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 (*T) Logf 

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 (*T) Name 

func (c *T) Name() string

Name returns the name of the running test or benchmark.

func (*T) Parallel 

func (t *T) Parallel()

Parallel signals that this test is to be run in parallel with (and only with) other parallel tests.

func (*T) Run 

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

Run runs f as a subtest of t called name. It reports whether f succeeded. Run will block until all its parallel subtests have completed.

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

func (*T) Skip 

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

Skip is equivalent to Log followed by SkipNow.

func (*T) SkipNow 

func (c *T) SkipNow()

SkipNow marks the test as having been skipped and stops its execution. 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 (*T) Skipf 

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

Skipf is equivalent to Logf followed by SkipNow.

func (*T) Skipped 

func (c *T) Skipped() bool

Skipped reports whether the test was skipped.

type TB

TB is the interface common to T and B. 

type TB interface {
    Error(args ...interface{})
    Errorf(format string, args ...interface{})
    Fail()
    FailNow()
    Failed() bool
    Fatal(args ...interface{})
    Fatalf(format string, args ...interface{})
    Log(args ...interface{})
    Logf(format string, args ...interface{})
    Name() string
    Skip(args ...interface{})
    SkipNow()
    Skipf(format string, args ...interface{})
    Skipped() bool
    // contains filtered or unexported methods
}

Subdirectories

Name Synopsis
..
iotest Package iotest implements Readers and Writers useful mainly for testing.
quick Package quick implements utility functions to help with black box testing.