いつも欲しくなったときにコードがどっか行くし、urllib.requestの使い方をしばしば誤るし、すぐ叩けず諦めてrequests入れて、とかやるのがめんどくさくなり、むしゃくしゃしてた。

標準ライブラリのみ
1つの関数で様々なMethodのHTTPを投げ分けたい
同一キーで2つ以上のパラメータは考えていない
multipart/form-dataも多分対応。ファイルデータは[filepath ,filename]という感じにリストにして与えると、ファイル読んで送る。[filepath]と長さ1の場合はbasenameで補完。（PerlのHTTP::Request::Commonの動きをまねた。）

import os
import json
import mimetypes

from urllib.parse import urlencode
from urllib.request import Request, urlopen, HTTPError


def str2byte(s):
    if type(s) == str:
        return s.encode()
    return s

def convert_multipart_data(data, boundry):
    CRLF = b"\r\n"

    boundry = str2byte(boundry)

    for key, val in data.items():
        key = str2byte(key)
        if type(val) == list:
            if len(val) == 1:
                filepath = val[0]
                name = None
            else:
                (filepath, name) = val
            if name is None:
                name = os.path.basename(filepath)
            name = str2byte(name)
            # size = os.path.getsize(filepath)
            mime = mimetypes.guess_type(filepath)[0].encode() or b'application/octet-stream'

            yield (b'--' + boundry + CRLF
                   + b'Content-Disposition: form-data; name="' + key + b'"; filename="' + name + b'"' + CRLF \
                   # + 'Content-Length: {0}'.format(size) + CRLF \
                   + b'Content-Type: ' + mime + CRLF \
                   + CRLF)
            with open(filepath, 'rb') as fp:
                for chunk in iter(lambda: fp.read(4096), b''):
                    yield chunk

            yield CRLF
        else:
            val = str2byte(val)
            yield (b'--' + boundry + CRLF
                   + b'Content-Disposition: form-data; name="' + key + b'"' + CRLF
                   + CRLF
                   + val + CRLF)

    yield (b'--' + boundry + b'--' + CRLF)
    return


def request_simple(url, data=None, headers={}, method=None, use_multipart_formdata=False):
    if method is None:
        method = 'GET' if data is None else 'POST'

    if method == 'POST' and use_multipart_formdata:
        boundary = 'bbboundaryyy'
        headers['Content-Type'] = 'multipart/form-data; boundary=' + boundary
        data = convert_multipart_data(data, boundary)
    # TODO: 直したい
    elif method == 'POST' and (headers.get('Content-Type') is None or headers['Content-Type'] == 'application/x-www-form-urlencoded'):
        headers['Content-Type'] = 'application/x-www-form-urlencoded'
        data = urlencode(data).encode()

    req = Request(url, data=data, headers=headers, method=method)

    try:
        with urlopen(req) as f:
            # 必要に応じて。bodyを返したり、そのまま返したり、jsonを解釈してdictで返したり
            return f.read()
#            return json.load(f.fp)
    except HTTPError as err:
        print(json.load(err.fp)) # 


# multipart/form-data
res = request_simple('http://localhost:8080/', headers={
    "Test-Header": "TEST"
}, data={
   'files[0]': 'cat-1.jpg', # value
    'files[1]': ['cat-4.jpg', 'image2.jpg'], # file
    'files[2]': ['cat-3.jpg', 'image.jpg'] # file
}, use_multipart_formdata=True)

print(res.decode('utf-8'))

# application/x-www-form-urlencoded
res = request_simple('http://localhost:8080/hoge?fuga&piyo', headers={
    "Test-Header": "TEST"
}, data={
   'files[0]': 'cat-1.jpg',
   'files[1]': 'cat-2.jpg'
})

print(res.decode('utf-8'))

ついでに、動作確認に使ってたHttp Serverとか(golang)

package main

import (
	"bytes"
	"fmt"
	"net/http"
	"strings"
)

func handler(w http.ResponseWriter, r *http.Request) {
	w.Header().Add("Content-Type", "text/plain")
	fmt.Fprintf(w, "%s %s %s\n", r.Method, r.RequestURI, r.Proto)

	for name, headers := range r.Header {
		for _, val := range headers {
			fmt.Fprintf(w, "%v: %v\n", name, val)
		}
	}
	fmt.Fprintf(w, "\n")

	contentType := r.Header.Get("Content-Type")
	if err := r.ParseMultipartForm(8192); err == nil {
		boundaryIndex := strings.Index(contentType, "boundary=")
		boundary := contentType[boundaryIndex+9:]

		for _, vals := range r.MultipartForm.File {
			fmt.Fprintf(w, "--%s\n", boundary)
			for _, val := range vals {
				for name, headers := range val.Header {
					for _, val := range headers {
						fmt.Fprintf(w, "%v: %v\n", name, val)
					}
				}
				fmt.Fprintf(w, "\n[data (%d byte)]\n", val.Size)
			}
		}
		fmt.Fprintf(w, "--%s--\n", boundary)
	}

	if err := r.ParseForm(); err == nil {
		for key, vals := range r.Form {
			for _, val := range vals {
				fmt.Fprintf(w, "%v=%v\n", key, val)
			}
		}
	}
	if r.Body != nil {
		bufbody := new(bytes.Buffer)
		bufbody.ReadFrom(r.Body)
		defer r.Body.Close()
		fmt.Fprintf(w, "%v", bufbody.String())
	}
}

func main() {
	http.HandleFunc("/", handler)
	http.ListenAndServe(":8080", nil)
}