How to Capture and Verify Log Output in Go Tests?

Accessing Logs in Go Tests

In Go testing, verifying the log output of a function can be challenging, especially when the function itself logs the error directly.

To capture the log output, a common solution is to redirect the default log output to a buffer before executing the function under test.

Example:

Consider the following readByte function:

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

To test the log output, we can use the following approach in the test file:

package main

import (
    "bytes"
    "testing"

    "github.com/stretchr/testify/assert"
)

func TestReadByte(t *testing.T) {
    var buf bytes.Buffer
    log.SetOutput(&buf)
    defer func() {
        log.SetOutput(os.Stderr)
    }()
    readByte()
    output := buf.String()
    assert.Contains(t, output, "Couldn't read first byte")
}

In this example, we:

• Create a buffer to capture the log output.
• Redirect the default log output to the buffer using log.SetOutput(&buf).
• Execute the readByte function.
• Restore the default log output to os.Stderr using defer func() to prevent affecting subsequent tests.
• Assert the expected log message using testify/assert.

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