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Testable Examples in Go

2015/05/07

Introduction

Godoc examples are snippets of Go code that are displayed as package documentation and that are verified by running them as tests. They can also be run by a user visiting the godoc web page for the package and clicking the associated "Run" button.

Having executable documentation for a package guarantees that the information will not go out of date as the API changes.

The standard library includes many such examples (see the strings package, for instance).

This article explains how to write your own example functions.

Examples are tests

Examples are compiled (and optionally executed) as part of a package's test suite.

As with typical tests, examples are functions that reside in a package's _test.go files. Unlike normal test functions, though, example functions take no arguments and begin with the word Example instead of Test.

The stringutil package is part of the Go example repository. Here's an example that demonstrates its Reverse function: 

package stringutil_test

import (
    "fmt"

    "github.com/golang/example/stringutil"
)

func ExampleReverse() {
    fmt.Println(stringutil.Reverse("hello"))
    // Output: olleh
}

This code might live in example_test.go in the stringutil directory.

Godoc will present this example alongside the Reverse function's documentation:

Running the package's test suite, we can see the example function is executed with no further arrangement from us: 

$ go test -v
=== RUN TestReverse
--- PASS: TestReverse (0.00s)
=== RUN: ExampleReverse
--- PASS: ExampleReverse (0.00s)
PASS
ok      github.com/golang/example/stringutil    0.009s

Output comments

What does it mean that the ExampleReverse function "passes"?

As it executes the example, the testing framework captures data written to standard output and then compares the output against the example's "Output:" comment. The test passes if the test's output matches its output comment.

To see a failing example we can change the output comment text to something obviously incorrect 

func ExampleReverse() {
    fmt.Println(stringutil.Reverse("hello"))
    // Output: golly
}

and run the tests again: 

$ go test
--- FAIL: ExampleReverse (0.00s)
got:
olleh
want:
golly
FAIL

If we remove the output comment entirely 

func ExampleReverse() {
    fmt.Println(stringutil.Reverse("hello"))
}

then the example function is compiled but not executed: 

$ go test -v
=== RUN TestReverse
--- PASS: TestReverse (0.00s)
PASS
ok      github.com/golang/example/stringutil    0.009s

Examples without output comments are useful for demonstrating code that cannot run as unit tests, such as that which accesses the network, while guaranteeing the example at least compiles.

Example function names

Godoc uses a naming convention to associate an example function with a package-level identifier. 

func ExampleFoo()     // documents the Foo function or type
func ExampleBar_Qux() // documents the Qux method of type Bar
func Example()        // documents the package as a whole

Following this convention, godoc displays the ExampleReverse example alongside the documentation for the Reverse function.

Multiple examples can be provided for a given identifier by using a suffix beginning with an underscore followed by a lowercase letter. Each of these examples documents the Reverse function: 

func ExampleReverse()
func ExampleReverse_second()
func ExampleReverse_third()

Larger examples

Sometimes we need more than just a function to write a good example.

For instance, to demonstrate the sort package we should show an implementation of sort.Interface. Since methods cannot be declared inside a function body, the example must include some context in addition to the example function.

To achieve this we can use a "whole file example." A whole file example is a file that ends in _test.go and contains exactly one example function, no test or benchmark functions, and at least one other package-level declaration. When displaying such examples godoc will show the entire file.

Here is a whole file example from the sort package: 

package sort_test

import (
    "fmt"
    "sort"
)

type Person struct {
    Name string
    Age  int
}

func (p Person) String() string {
    return fmt.Sprintf("%s: %d", p.Name, p.Age)
}

// ByAge implements sort.Interface for []Person based on
// the Age field.
type ByAge []Person

func (a ByAge) Len() int           { return len(a) }
func (a ByAge) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
func (a ByAge) Less(i, j int) bool { return a[i].Age < a[j].Age }

func Example() {
    people := []Person{
        {"Bob", 31},
        {"John", 42},
        {"Michael", 17},
        {"Jenny", 26},
    }

    fmt.Println(people)
    sort.Sort(ByAge(people))
    fmt.Println(people)

    // Output:
    // [Bob: 31 John: 42 Michael: 17 Jenny: 26]
    // [Michael: 17 Jenny: 26 Bob: 31 John: 42]
}

A package can contain multiple whole file examples; one example per file. Take a look at the sort package's source code to see this in practice.

Conclusion

Godoc examples are a great way to write and maintain code as documentation. They also present editable, working, runnable examples your users can build on. Use them!

Andrew Gerrand 编写

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