Golang: Change the proxy net/http uses during execution.

proxy.go

package main

import (
	"fmt"
	"io/ioutil"
	"net/http"
	"net/url"
)

// Go's net/http gets (and caches) the environment variable HTTP_PROXY
// to use as a proxy for all its communications.
// If you try os.Setenv("HTTP_PROXY", "newproxyserver:8080") you'll
// notice that it won't change due to net/http caching the initial value.
// This is how to change the proxy during execution.

func main() {
	proxyURL, _ := url.Parse("http://proxyserver:80")
	client := http.Client{
		Transport: &http.Transport{
			Proxy: http.ProxyURL(proxyURL),
		},
	}
	resp, err := client.Get("http://example.com")
	if err != nil {
		panic(err)
	}
	defer resp.Body.Close()
	bs, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		panic(err)
	}
	fmt.Print(string(bs[:]))
}