How to Test Logged Output in Go Tests Without Modifying the Function?

Testing Logged Output in Go Tests

In Go, unit tests often involve verifying the output or logs generated by the tested function. Suppose we have a function that logs errors in certain scenarios.

The Problem:

Given the following function readByte that logs an error message:

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

How can we test the output error in readByte using a go test without modifying the function itself?

The Solution:

To capture the log output, we can redirect the output of the log package to a temporary buffer during the test and then assert the contents of the buffer.

Example Code:

In the readbyte_test.go file:

package main

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

func readByte( /*...*/ ) {
    // ...
    err := io.EOF // Force an error
    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.Logf("Output: %s", buf.String())
}

Example Output:

$ go test -v readbyte_test.go
=== RUN   TestReadByte
--- PASS: TestReadByte (0.00s)
        readbyte_test.go:30: Output: ERROR
        Couldn't read first byte
PASS
ok      command-line-arguments  0.004s
$ 

In this example, the outputting error messages can now be tested and verified using Go's built-in testing tools without altering the original readByte function.

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