代码如下:
 
import tornado.ioloop
import tornado.web
from tornado.httpclient import HTTPClient, AsyncHTTPClient

class MainHandler(tornado.web.RequestHandler):
    # 同步
    def get(self):
        h_c = HTTPClient()
        res = h_c.fetch("http://www.baidu.com")
        # print(res)
        # pass
        self.write("Hello, world")


class TestHandler(tornado.web.RequestHandler):
    # 异步
    async def get(self):
        http_client = AsyncHTTPClient()
        try:
            res = await http_client.fetch("http://www.baidu.com")
        except Exception as e:
            print("Error: %s" % e)
        else:
            pass
        self.write("Hello, world1")


def make_app():
    return tornado.web.Application([
        (r"/test", TestHandler),
        (r"/", MainHandler),
    ])


if __name__ == "__main__":
    app = make_app()
    app.listen(8888)
    tornado.ioloop.IOLoop.current().start()
 
浏览器打开:
 
http://127.0.0.1:8888/
 
完整报错:
 
ERROR:tornado.application:Uncaught exception GET / (127.0.0.1)
 HTTPServerRequest(protocol='http', host='127.0.0.1:8888', method='GET', uri='/', version='HTTP/1.1', remote_ip='127.0.0.1')
 Traceback (most recent call last):
   File "/home/appleyuchi/anaconda3/envs/Python3.6/lib/python3.6/site-packages/tornado/web.py", line 1590, in _execute
     result = method(*self.path_args, **self.path_kwargs)
   File "test.py", line 8, in get
     h_c = HTTPClient()
   File "/home/appleyuchi/anaconda3/envs/Python3.6/lib/python3.6/site-packages/tornado/httpclient.py", line 95, in __init__
     gen.coroutine(lambda: async_client_class(**kwargs)))
   File "/home/appleyuchi/anaconda3/envs/Python3.6/lib/python3.6/site-packages/tornado/ioloop.py", line 571, in run_sync
     self.start()
   File "/home/appleyuchi/anaconda3/envs/Python3.6/lib/python3.6/site-packages/tornado/platform/asyncio.py", line 132, in start
     self.asyncio_loop.run_forever()
   File "/home/appleyuchi/anaconda3/envs/Python3.6/lib/python3.6/asyncio/base_events.py", line 432, in run_forever
     'Cannot run the event loop while another loop is running')
 RuntimeError: Cannot run the event loop while another loop is running
 ERROR:tornado.access:500 GET / (127.0.0.1) 3.29ms
 
根据官方文档:
 
Changed in version 5.0: Due to limitations in asyncio, it is no longer possible to use the synchronous HTTPClient while an IOLoop is running. Use AsyncHTTPClient instead.
 
解决方案:
 
Tornado同步api和异步api混写一例

我是在TensorFlow的教程中发现的报错：
 
 执行这行代码会报错：Cannot run the event loop while another loop is running
 
解决方法：
 在 jupyter-notebook 的一个单元格中输入：
 
!pip install nest_asyncio
import nest_asyncio
nest_asyncio.apply()

 1.这里只是写了个简单的demo1，来记录为什么报错，见下面代码
 
# demo1.py

import tornado.web
import tornado.ioloop
import tornado.httpserver
import tornado.options
import tornado.httpclient


from tornado.options import define, options
define("port", default=8989, help="run on the given port", type=int)


class IndexHandler(tornado.web.RequestHandler):
    def get(self):
        client = tornado.httpclient.HTTPClient()
        response = client.fetch("https://www.baidu.com")
        print(response.body)
        self.write("ok")


if __name__ == '__main__':

    tornado.options.parse_command_line()
    app = tornado.web.Application(
        handlers=[
            (r"/", IndexHandler)
        ],
        debug=True,

    )
    http_server = tornado.httpserver.HTTPServer(app)
    http_server.listen(options.port)
    tornado.ioloop.IOLoop.instance().start()
 
正是在tornado的服务里面（存在ioloop循环）使用了HTTPClient，所以才报错了，这是因为在tornado 5.0以后只能在tornado服务中使用异步客户端AsyncHTTPClient，
 
解决使用同步客户端的问题是可以使用python的requests包
 
2.如果像下面这样，直接使用是没有问题的（不使用ioloop循环）
 
def h():
    http_client = tornado.httpclient.HTTPClient()
    try:
        response = http_client.fetch("https://www.baidu.com")
        print(response.body)
    except tornado.httpclient.HTTPError as e:
        # HTTPError is raised for non-200 responses; the response
        # can be found in e.response.
        print("Error: " + str(e))
    except Exception as e:
        # Other errors are possible, such as IOError.
        print("Error: " + str(e))
    http_client.close()

h()
 
3.如果想在tornado（版本5.0以后）服务中使用客户端，那么只能使用AsyncHTTPClient，如下
 
import tornado.web
import tornado.ioloop
import tornado.httpserver
import tornado.options
import tornado.httpclient


from tornado.options import define, options
define("port", default=8989, help="run on the given port", type=int)


class IndexHandler(tornado.web.RequestHandler):
    async def get(self):
        client = tornado.httpclient.AsyncHTTPClient()
        response = await client.fetch("https://www.baidu.com")
        print(response.body)
        self.write("ok")


if __name__ == '__main__':

    tornado.options.parse_command_line()
    app = tornado.web.Application(
        handlers=[
            (r"/", IndexHandler)
        ],
        debug=True,

    )
    http_server = tornado.httpserver.HTTPServer(app)
    http_server.listen(options.port)
    tornado.ioloop.IOLoop.instance().start()
 
 

 
SAP LOOP WHILE中修改内表数据
 
场景：
解决方法：


 
场景：
 
在Loop中用工作区修改另外一个相同的内表时，发生DUMP：TABLE_ILLEGAL_STATEMENT
短文本描述：Error in an ABAP statement (processing an internal table).
使用语法：MODIFY lt_sflight3 FROM WA_SFLIGHT .
 
 
 
解决方法：
 
使用语法3中的语句：
 
MODIFY lt_sflight1 FROM wa_sflight.
MODIFY lt_sflight2
MODIFY lt_sflight3 FROM WA_SFLIGHT TRANSPORTING CARRID WHERE CARRID = ‘AA’ AND FLDATE = ‘20200423’. "可在LOOP WHILE中修改其他内表的数据
 
DATA: lt_sflight1 TYPE TABLE OF SFLIGHT,
      lt_sflight2 TYPE TABLE OF SFLIGHT WITH HEADER LINE,
      lt_sflight3 TYPE TABLE OF SFLIGHT.

SELECT * FROM SFLIGHT INTO TABLE lt_sflight1 UP TO 3 ROWS.
SELECT * FROM SFLIGHT INTO TABLE lt_sflight2 UP TO 3 ROWS.
SELECT * FROM SFLIGHT INTO TABLE lt_sflight3 UP TO 3 ROWS.

  LOOP AT lt_sflight1 INTO DATA(wa_sflight).
    wa_sflight-CARRID = 'XX'.
    MODIFY lt_sflight1 FROM wa_sflight.
    MODIFY lt_sflight3 FROM WA_SFLIGHT TRANSPORTING CARRID WHERE CARRID = 'AA' AND CONNID = '0017' AND FLDATE = '20200423'.
  ENDLOOP.

  LOOP AT lt_sflight2.
    lt_sflight2-CARRID = 'XY'.
    MODIFY lt_sflight2.
    MODIFY lt_sflight3 FROM lt_sflight2 TRANSPORTING CARRID WHERE CARRID = 'AA' AND CONNID = '0017' AND FLDATE = '20200525'.
  ENDLOOP.

cl_demo_output=>display( lt_sflight1 ).
cl_demo_output=>display( lt_sflight2[] ).
cl_demo_output=>display( lt_sflight3 ).