The Go Blog

Using Subtests and Sub-benchmarks

Marcel van Lohuizen
3 October 2016

Introduction

In Go 1.7, the testing package introduces a Run method on the T and B types that allows for the creation of subtests and sub-benchmarks. The introduction of subtests and sub-benchmarks enables better handling of failures, fine-grained control of which tests to run from the command line, control of parallelism, and often results in simpler and more maintainable code.

Table-driven tests basics

Before digging into the details, let's first discuss a common way of writing tests in Go. A series of related checks can be implemented by looping over a slice of test cases:

func TestTime(t *testing.T) {
    testCases := []struct {
        gmt  string
        loc  string
        want string
    }{
        {"12:31", "Europe/Zuri", "13:31"},     // incorrect location name
        {"12:31", "America/New_York", "7:31"}, // should be 07:31
        {"08:08", "Australia/Sydney", "18:08"},
    }
    for _, tc := range testCases {
        loc, err := time.LoadLocation(tc.loc)
        if err != nil {
            t.Fatalf("could not load location %q", tc.loc)
        }
        gmt, _ := time.Parse("15:04", tc.gmt)
        if got := gmt.In(loc).Format("15:04"); got != tc.want {
            t.Errorf("In(%s, %s) = %s; want %s", tc.gmt, tc.loc, got, tc.want)
        }
    }
}

This approach, commonly referred to as table-driven tests, reduces the amount of repetitive code compared to repeating the same code for each test and makes it straightforward to add more test cases.

Table-driven benchmarks

Before Go 1.7 it was not possible to use the same table-driven approach for benchmarks. A benchmark tests the performance of an entire function, so iterating over benchmarks would just measure all of them as a single benchmark.

A common workaround was to define separate top-level benchmarks that each call a common function with different parameters. For instance, before 1.7 the strconv package's benchmarks for AppendFloat looked something like this:

func benchmarkAppendFloat(b *testing.B, f float64, fmt byte, prec, bitSize int) {
    dst := make([]byte, 30)
    b.ResetTimer() // Overkill here, but for illustrative purposes.
    for i := 0; i < b.N; i++ {
        AppendFloat(dst[:0], f, fmt, prec, bitSize)
    }
}

func BenchmarkAppendFloatDecimal(b *testing.B) { benchmarkAppendFloat(b, 33909, 'g', -1, 64) }
func BenchmarkAppendFloat(b *testing.B)        { benchmarkAppendFloat(b, 339.7784, 'g', -1, 64) }
func BenchmarkAppendFloatExp(b *testing.B)     { benchmarkAppendFloat(b, -5.09e75, 'g', -1, 64) }
func BenchmarkAppendFloatNegExp(b *testing.B)  { benchmarkAppendFloat(b, -5.11e-95, 'g', -1, 64) }
func BenchmarkAppendFloatBig(b *testing.B)     { benchmarkAppendFloat(b, 123456789123456789123456789, 'g', -1, 64) }
...

Using the Run method available in Go 1.7, the same set of benchmarks is now expressed as a single top-level benchmark:

func BenchmarkAppendFloat(b *testing.B) {
    benchmarks := []struct{
        name    string
        float   float64
        fmt     byte
        prec    int
        bitSize int
    }{
        {"Decimal", 33909, 'g', -1, 64},
        {"Float", 339.7784, 'g', -1, 64},
        {"Exp", -5.09e75, 'g', -1, 64},
        {"NegExp", -5.11e-95, 'g', -1, 64},
        {"Big", 123456789123456789123456789, 'g', -1, 64},
        ...
    }
    dst := make([]byte, 30)
    for _, bm := range benchmarks {
        b.Run(bm.name, func(b *testing.B) {
            for i := 0; i < b.N; i++ {
                AppendFloat(dst[:0], bm.float, bm.fmt, bm.prec, bm.bitSize)
            }
        })
    }
}

Each invocation of the Run method creates a separate benchmark. An enclosing benchmark function that calls a Run method is only run once and is not measured.

The new code has more lines of code, but is more maintainable, more readable, and consistent with the table-driven approach commonly used for testing. Moreover, common setup code is now shared between runs while eliminating the need to reset the timer.

Table-driven tests using subtests

Go 1.7 also introduces a Run method for creating subtests. This test is a rewritten version of our earlier example using subtests:

func TestTime(t *testing.T) {
    testCases := []struct {
        gmt  string
        loc  string
        want string
    }{
        {"12:31", "Europe/Zuri", "13:31"},
        {"12:31", "America/New_York", "7:31"},
        {"08:08", "Australia/Sydney", "18:08"},
    }
    for _, tc := range testCases {
        t.Run(fmt.Sprintf("%s in %s", tc.gmt, tc.loc), func(t *testing.T) {
            loc, err := time.LoadLocation(tc.loc)
            if err != nil {
                t.Fatal("could not load location")
            }
            gmt, _ := time.Parse("15:04", tc.gmt)
            if got := gmt.In(loc).Format("15:04"); got != tc.want {
                t.Errorf("got %s; want %s", got, tc.want)
            }
        })
    }
}

The first thing to note is the difference in output from the two implementations. The original implementation prints:

--- FAIL: TestTime (0.00s)
    time_test.go:62: could not load location "Europe/Zuri"

Even though there are two errors, execution of the test halts on the call to Fatalf and the second test never runs.

The implementation using Run prints both:

--- FAIL: TestTime (0.00s)
    --- FAIL: TestTime/12:31_in_Europe/Zuri (0.00s)
        time_test.go:84: could not load location
    --- FAIL: TestTime/12:31_in_America/New_York (0.00s)
        time_test.go:88: got 07:31; want 7:31

Fatal and its siblings causes a subtest to be skipped but not its parent or subsequent subtests.

Another thing to note is the shorter error messages in the new implementation. Since the subtest name uniquely identifies the subtest there is no need to identify the test again within the error messages.

There are several other benefits to using subtests or sub-benchmarks, as clarified by the following sections.

Running specific tests or benchmarks

Both subtests and sub-benchmarks can be singled out on the command line using the -run or -bench flag. Both flags take a slash-separated list of regular expressions that match the corresponding parts of the full name of the subtest or sub-benchmark.

The full name of a subtest or sub-benchmark is a slash-separated list of its name and the names of all of its parents, starting with the top-level. The name is the corresponding function name for top-level tests and benchmarks, and the first argument to Run otherwise. To avoid display and parsing issues, a name is sanitized by replacing spaces with underscores and escaping non-printable characters. The same sanitizing is applied to the regular expressions passed to the -run or -bench flags.

A few examples:

Run tests that use a timezone in Europe:

$ go test -run=TestTime/"in Europe"
--- FAIL: TestTime (0.00s)
    --- FAIL: TestTime/12:31_in_Europe/Zuri (0.00s)
        time_test.go:85: could not load location

Run only tests for times after noon:

$ go test -run=Time/12:[0-9] -v
=== RUN   TestTime
=== RUN   TestTime/12:31_in_Europe/Zuri
=== RUN   TestTime/12:31_in_America/New_York
--- FAIL: TestTime (0.00s)
    --- FAIL: TestTime/12:31_in_Europe/Zuri (0.00s)
        time_test.go:85: could not load location
    --- FAIL: TestTime/12:31_in_America/New_York (0.00s)
        time_test.go:89: got 07:31; want 7:31

Perhaps a bit surprising, using -run=TestTime/New_York won't match any tests. This is because the slash present in the location names is treated as a separator as well. Instead use:

$ go test -run=Time//New_York
--- FAIL: TestTime (0.00s)
    --- FAIL: TestTime/12:31_in_America/New_York (0.00s)
        time_test.go:88: got 07:31; want 7:31

Note the // in the string passed to -run. The / in time zone name America/New_York is handled as if it were a separator resulting from a subtest. The first regular expression of the pattern (TestTime) matches the top-level test. The second regular expression (the empty string) matches anything, in this case the time and the continent part of the location. The third regular expression (New_York) matches the city part of the location.

Treating slashes in names as separators allows the user to refactor hierarchies of tests without the need to change the naming. It also simplifies the escaping rules. The user should escape slashes in names, for instance by replacing them with backslashes, if this poses a problem.

A unique sequence number is appended to test names that are not unique. So one could just pass an empty string to Run if there is no obvious naming scheme for subtests and the subtests can easily be identified by their sequence number.

Setup and Tear-down

Subtests and sub-benchmarks can be used to manage 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) {
        if !test(foo{B:1}) {
            t.Fail()
        }
    })
    // <tear-down code>
}

The setup and tear-down code will run if any of the enclosed subtests are run and will run at most once. This applies even if any of the subtests calls Skip, Fail, or Fatal.

Control of Parallelism

Subtests allow fine-grained control over parallelism. To understand how to use subtests in the way it is important to understand the semantics of parallel tests.

Each test is associated with a test function. A test is called a parallel test if its test function calls the Parallel method on its instance of testing.T. A parallel test never runs concurrently with a sequential test and its execution is suspended until its calling test function, that of the parent test, has returned. The -parallel flag defines the maximum number of parallel tests that can run in parallel.

A test blocks until its test function returns and all of its subtests have completed. This means that the parallel tests that are run by a sequential test will complete before any other consecutive sequential test is run.

This behavior is identical for tests created by Run and top-level tests. In fact, under the hood top-level tests are implemented as subtests of a hidden master test.

Run a group of tests in parallel

The above semantics allows for running a group of tests in parallel with each other but not with other parallel tests:

func TestGroupedParallel(t *testing.T) {
    for _, tc := range testCases {
        tc := tc // capture range variable
        t.Run(tc.Name, func(t *testing.T) {
            t.Parallel()
            if got := foo(tc.in); got != tc.out {
                t.Errorf("got %v; want %v", got, tc.out)
            }
            ...
        })
    }
}

The outer test will not complete until all parallel tests started by Run have completed. As a result, no other parallel tests can run in parallel to these parallel tests.

Note that we need to capture the range variable to ensure that tc gets bound to the correct instance.

Cleaning up after a group of parallel tests

In the previous example we used the semantics to wait on a group of parallel tests to complete before commencing other tests. The same technique can be used to clean up after a group of parallel tests that share common resources:

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

The behavior of waiting on a group of parallel tests is identical to that of the previous example.

Conclusion

Go 1.7's addition of subtests and sub-benchmarks allows you to write structured tests and benchmarks in a natural way that blends nicely into the existing tools. One way to think about this is that earlier versions of the testing package had a 1-level hierarchy: the package-level test was structured as a set of individual tests and benchmarks. Now that structure has been extended to those individual tests and benchmarks, recursively. In fact, in the implementation, the top-level tests and benchmarks are tracked as if they were subtests and sub-benchmarks of an implicit master test and benchmark: the treatment really is the same at all levels.

The ability for tests to define this structure enables fine-grained execution of specific test cases, shared setup and teardown, and better control over test parallelism. We are excited to see what other uses people find. Enjoy.

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