How to measure test coverage in Go

code-coverage, go, testing, unit-testing

Solution

Note that Go 1.2 (Q4 2013, rc1 is available) will now display test coverage results:

One major new feature of `go test` is that it can now compute and, with help from a new, separately installed "`go tool cover`" program, display test coverage results.

The `cover` tool is part of the `go.tools` subrepository. It can be installed by running

$ go get golang.org/x/tools/cmd/cover

The cover tool does two things.

First, when "`go test`" is given the `-cover` flag, it is run automatically to rewrite the source for the package and insert instrumentation statements. The test is then compiled and run as usual, and basic coverage statistics are reported:

$ go test -coverprofile fmtcoverage.html fmt ok fmt 0.060s coverage: 91.4% of statements $

Second, for more detailed reports, different flags to "go test" can create a coverage profile file, which the cover program, invoked with "`go tool cover`", can then analyze.

Frank Shearar mentions:

The latest versions of Go (2013/09/19) use:

go test -coverprofile <filename> <package name>

Details on how to generate and analyze coverage statistics can be found by running the commands

$ go help testflag
$ go tool cover -help

Ivan Black mentions in the comments:

`go test -coverprofile cover.out` and then `go tool cover -html=cover.out` opens `cover.out` in your default browser

I don't even want to wait for the browser to open, so I defined this alias:

alias gc=grep -v -e " 1$" cover.out

That I just type `gc`, and have a list of all the lines not yet covered (here: with a `coverage.out` line not ending with " `1`").

Update 2022, possibly for Go 1.19

proposal: extend Go's code coverage testing to include applications

While the existing "`go test`" based coverage workflow will continue to be supported, the proposal is to add coverage as a new build mode for "`go build`".

In the same way that users can build a race-detector instrumented executable using "`go build -race`", it will be possible to build a coverage-instrumented executable using "`go build -cover`".

Merging coverage profiles produced in different GOOS/GOARCH environments will be supported.

Update March 2023, Go 1.20: "Code coverage for Go integration tests" shows that you can now build coverage-instrumented programs using “go build -cover”, then feed these instrumented binaries into an integration test to extend the scope of coverage testing.

Problem

Original source