An Introduction to Golang Unit Testing

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Unit testing verifies the functionality of a specific section of code in isolation. In the Go language, go test is the built-in command that runs unit tests, example functions, and benchmarks; does code profiling, and performs code coverage analysis. This guide provides an introduction to unit testing in Go.

Go Test Built-in Commands

The go test command automates testing packages named by import paths. The syntax used to run Go tests is as follows:

go test [build/test flags] [packages] [build/test flags & test binary flags]

Running your tests prints a summary of the test results as shown below:

ok   archive/tar   0.011s
FAIL archive/zip   0.022s
ok   compress/gzip 0.033s
...

A detailed output is provided for any check that fails.

go test recompiles each package and any files with names matching the file pattern *_test.go. These additional files can contain test functions, benchmark functions, and example functions. Each listed package causes the execution of a separate test binary. Files whose names begin with and underscore (_) (including _test.go) or a dot (.) are ignored.

Test files that declare a package with the suffix _test are compiled as a separate package. They are then linked and run with the main test binary. Go’s built-in test runner ignores a directory named testdata, making it available to store ancillary data needed by the tests.

As part of building a test binary, go test runs go vet on the package and its test source files to identify significant problems. If go vet finds any problems, go test reports those and does not run the test binary. Only a high-confidence subset of the default go vet checks are used. The subset of checks are: atomic, bool, buildtags, errorsas, ifaceassert, nilfunc, printf, and stringintconv. You can view the documentation for these and other vet tests by issuing the go doc cmd/vet command. To disable the running of go vet, use the -vet=off flag.

All test output and summary lines are printed to Go’s standard output. Go’s standard error is reserved for printing errors while building the tests.

Go Test Modes

go test runs in two different modes. The first, called local directory mode, occurs when go test is invoked with no package arguments. In this mode, go test compiles the package sources and tests found in the current directory and then, runs the resulting test binary. Caching is also disabled in this mode. After the package test finishes, go test prints a summary line showing the test status (ok or FAIL), package name, and elapsed time.

The second mode, called package list mode, occurs when go test is invoked with explicit package arguments. For example, the following commands trigger package list mode: go test math, go test ./..., and go test .. In this mode, go test compiles and tests each of the packages listed on the command line. When a package test passes, go test prints only the final ok summary line. When a package test fails, go test prints the full test output. When invoked with the -bench or -v flag, go test prints the full output even for package tests that pass. This is done in order to display the requested benchmark results or verbose logging. When all listed package tests finish, and their output is printed, go test prints a final FAIL status if any package test has failed.

Go Test Caching

In package list mode, go test only caches successful package test results to avoid rerunning tests that have already passed. When the result of a test can be recovered from the cache, go test displays the previous output instead of running the test binary again. When this happens, go test prints (cached) in place of the elapsed time in the summary line.

In order for go test to use a cached test result, the run must involve the same test binary and the command-line flags must come from a restricted set of cacheable test flags. The restricted set of command-line flags include -cpu, -list, -parallel, -run, -short, and -v. When a run of go test has any test or non-test flags outside this set, the result is not cached. To disable test caching, use any test flag or argument other than the cacheable flags. The idiomatic way to disable test caching explicitly is to use -count=1. Tests that rely on environment variables are only cacheable if the environment variables remain unchanged. Similarly, tests that open files within the package’s source root, are cached only if the opened files remain unchanged. A successful package test result is cached and reused regardless of the -timeout setting.

Go Test Command-Line Flags

In addition to the build flags, some additional go test flags are the following:

  • -args: Pass the remainder of the command line (everything after -args) to the test binary, uninterpreted and unchanged. Because this flag consumes the remainder of the command line, the package list (if present) must appear before this flag.

  • -c : Compile the test binary to pkg.test, but do not run it (where pkg is the last element of the package’s import path). The filename can be changed with the -o flag.

  • -exec: Run the test binary using xprog. The behavior is the same as in go run. See go help run for details.

  • -i: Install packages that are dependencies of the test. Do not run the test. The -i flag is deprecated. Compiled packages are cached automatically.

  • -json: Convert test output to JSON suitable for automated processing. See go doc test2json for the encoding details.

  • -o file: Compile the test binary to the named file. The test still runs (unless -c or -i is specified).

For more information about build flags, run the go help build command. To access information about specifying packages, run the go help packages command.

Go Package Testing

The standard Go language testing package provides support for automated testing of Go code. It is intended to be used with the go test command. As with everything else in the Golang, the testing package design is driven by a minimalist philosophy. For example, test result comparisons using the testing package are implemented by writing ordinary Go code with comparison operators. This is in contrast to using assertions as you would in JUnit or NUnit.

The following example test function is included in the testing package documentation. It’s not testable as given (it requires a matching Abs() function for integers), but it demonstrates the syntax required.

File: example.go
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import "testing"

func TestAbs(t *testing.T) {
    got := Abs(-1)
    if got != 1 {
        t.Errorf("Abs(-1) = %d; want 1", got)
    }
}

Go test functions can easily be extended to perform table-driven tests to cover many cases. This is done from a single function with the help of a for loop. You can also take advantage of Go interfaces and functional programming to implement mocks.

In general, the Go way is to rely on the built-in functionality of the language as much as possible. In practice, writing tests in the basic testing package can be somewhat repetitive. Writing tests in this way is also different from tests written in other programming languages. If you find this is the case for you, there are several third-party Golang testing frameworks you can use. These include:

  • Testify: Assertion and mock helper functions.
  • gocheck: Assertion helper functions.
  • gopwt: Power assertion helper functions.
  • go-testdeep: Deep comparison helper functions.
  • Ginkgo and Gomega: A heavyweight behavior-driven development (BDD) testing framework and assertion helpers.
  • Goblin: A Mocha-like BDD testing framework.
  • GoConvey: BDD testing framework with web UI.

Example Unit Test

This section expands on an integer Abs() function and the sample unit test from the Go testing package documentation. Start by creating a directory called abs_test under your Go source code directory. In the examples below, the GOPATH is set to ~/work, and the directory created for this project is ~/work/src/github.com/meheller/abs_test.

  1. Open a terminal window, change (cd) to the directory you created and initialize the project.

    go mod init

    This creates a file named go.mod. The file includes content similar to the example below:

    File: go.mod
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    module github.com/meheller/abs_test
    
    go 1.16
        
    
  2. Use the cat command to print the contents of your go.mod file.

  3. Create a new abs.go file in abs_test directory, and insert the following contents:

    File: abs.go
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    package abs
    
    func Abs(x int) (res int) {
        if x >= 0 {
            return x
        }
        return -x
    }
    
        
    
  4. Save the file and create another file, abs_test.go, in the same directory as the unit tests. Use the single test from the documentation, with a matching package tag added as show below:

    File: abs_test.go
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    package abs
    
    import (
        "testing"
    )
    
    func TestAbs(t *testing.T) {
        got := Abs(-1)
        if got != 1 {
            t.Errorf("Abs(-1) = %d; want 1", got)
        }
    }
        
    
  5. Go back to your terminal and make sure all three files are present.

     ls
    
    abs.go abs_test.go go.mod
  6. Now, run your tests using the following command:

     go test -v
    

    You should see a similar result:

    === RUN   TestAbs
    --- PASS: TestAbs (0.00s)
    PASS
    ok  	github.com/meheller/abs_test 0.006s

You can create more tests in the abs_test.go file. One useful exercise is to use a table of input values and expected output values. Then, run your tests in a for loop to make sure that the function handles all cases properly. (-1, 0, 1) is the minimum starting set of inputs. Writing unit tests for all your functions is a best practice. Table-driven tests help you to cover all the cases without copying and pasting a lot of test code.

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