How to Capture and Test Log Output in Go Tests?

Accessing Log Output in Go Tests

In Go, the log package provides a convenient way to log messages. However, when writing tests that interact with code that generates log messages, testing these messages can become a challenge.

Consider the following function readByte that logs an error when encountered:

func readByte(/*...*/) {
    // ...
    if err != nil {
        fmt.Println("ERROR")
        log.Print("Couldn't read first byte")
        return
    }
    // ...
}

In a corresponding test file, you may attempt to verify the error output using:

c.Assert(OUTPUT, check.Matches, "teste")

However, this approach will not work since the OUTPUT variable does not have access to the log buffer. To resolve this, you can employ a method to capture the log output during the test.

Capturing Log Output

One approach to capture log output is to temporarily redirect the standard output where the log is printed. This can be achieved through the log.SetOutput function, as demonstrated in the following example:

func TestReadByte(t *testing.T) {
    var buf bytes.Buffer
    // Update output to write to the buffer
    log.SetOutput(&buf)
    // Reset output to original after test
    defer func() {
        log.SetOutput(os.Stderr)
    }()
    readByte()
    // Verify the buffer contains the expected log message
    t.Log(buf.String())
}

Running the test will produce an output similar to:

ERROR
Couldn't read first byte

This demonstrates how you can effectively access and verify log output within your Go tests, allowing you to thoroughly test the behavior of code that generates logs.

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