How to Capture and Inspect Logs in Go Tests?

Capturing and Inspecting Logs in Go Tests

To verify the logs generated by a function under test, it's essential to capture and inspect them. This can be achieved by redirecting the output and comparing it with the expected values.

In the provided example, the readByte function prints the log messages to the standard output. To capture these messages in the test, we can utilize a bytes.Buffer as shown below:

package main

import (
    "bytes"
    "fmt"
    "io"
    "log"
    "os"
    "testing"
)

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

func TestReadByte(t *testing.T) {
    var buf bytes.Buffer
    log.SetOutput(&buf)
    defer func() {
        log.SetOutput(os.Stderr)
    }()
    readByte()
    t.Log(buf.String())
}

Explanation:

• The log.SetOutput(&buf) line redirects the log output to the buf variable.
• The defer log.SetOutput(os.Stderr) ensures that the log output is restored to its original destination after the test runs.
• The t.Log(buf.String()) line prints the captured logs to the test log.

By examining the contents of buf, you can verify if the expected logs were generated by the readByte function. This approach allows you to test log output without modifying the code of the function under test.

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