http request with context in Go

main.go

package main

import (
	"context"
	"fmt"
	"log"
	"net/http"
	"time"
)

func main() {
	req, err := http.NewRequest("GET", "http://www.yahoo.co.jp", nil)
	if err != nil {
		log.Fatalf("%v", err)
	}

	ctx, cancel := context.WithTimeout(req.Context(), 1*time.Millisecond)
	defer cancel()

	req = req.WithContext(ctx)

	client := http.DefaultClient
	res, err := client.Do(req)
	if err != nil {
		log.Fatalf("%v", err)
	}

	fmt.Printf("%v\n", res.StatusCode)
}

There are many examples that use the parent context from context.Background(). But in this example, I see the parent is using r.Context() from request instead. So, which one that should I use?

it's actually the same. When he initialized the request with http.NewRequest(), per the documentation, the context of the request is the background context:

// NewRequest wraps NewRequestWithContext using the background context.