downloaserver.py：

#!/usr/bin/env python
# coding=utf-8
# 服务器端，下载图像.jpg

import flask
import os

app = flask.Flask(__name__)

@app.route("/",methods=["GET","POST"])
def index():
    # print(flask.request.values)
    if "fileName" not in flask.request.values:
        return "图像.jpg"
    else:
        data = b""
        try:
            fileName = flask.request.values.get("fileName")
            if fileName != "" and os.path.exists(fileName):
                with open(fileName,'rb')as f:
                    data = f.read()
        except Exception as e:
            data = str(e).encode()
        return data

if __name__ == "__main__":
    app.run()

downloadclient.py：

#!/usr/bin/env python
# coding=utf-8
# 客户端下载

import urllib.parse
import urllib.request
import time

url = "http://127.0.0.1:5000"
try:
    request = urllib.request.urlopen(url)
    response = request.read()
    fileName = response.decode()
    # GET方法下载
    # data = urllib.request.urlopen(url+"?fileName="+urllib.parse.quote(fileName))
    # POS方法下载
    filedata= "fileName="+urllib.parse.quote(fileName)
    data = urllib.request.urlopen(url,data=filedata.encode())
    data = data.read()
    print("准备下载文件%s,大小为%s字节："%(fileName,len(data)))
    time.sleep(3)
    with open("download"+fileName,'wb')as f:
        f.write(data)
    print("下载完成,下载的文件大小%s字节"%len(data))

    # 另一种下载图像方法，简便实用
    # print("准备下载文件"+fileName)
    # time.sleep(3)
    # urllib.request.urlretrieve(url+"?fileName="+urllib.parse.quote(fileName),"download"+fileName)
    # print("下载完成！")

except Exception as e:
    print(e)