从 Linux 上的 Python 脚本访问 Popen web 服务器

Question

I have this script and try to run it on CentOS 7 (though I'm fairly sure the distro doesn't matter too much):

from subprocess import Popen, DEVNULL
from urllib import request

p = Popen(['python', '-m', 'http.server'], cwd='.', stderr=DEVNULL, stdout=DEVNULL)
try:
   r = request.urlopen('http://localhost:8000')
   print(r.read())
finally:
   p.terminate()
   p.wait()

I know the Popen is correct in principle, because if I run this script instead, I can read from the server with something like curl localhost:8000 :

from subprocess import Popen, DEVNULL
from urllib import request
from time import sleep

p = Popen(['python', '-m', 'http.server'], cwd='.', stderr=DEVNULL, stdout=DEVNULL)
try:
   sleep(30)
finally:
   p.terminate()
   p.wait()

I also know the .urlopen() command should work, since I can run a similar web server from the command line, then open an interactive Python session and run the same two lines:

r = request.urlopen('http://localhost:8000')
print(r.read())

And it will print the expected page.

My question: why would running the first script result in connection errors?

Specifically, I get these errors:

Traceback (most recent call last):
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/urllib/request.py", line 1342, in do_open
    h.request(req.get_method(), req.selector, req.data, headers,
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/http/client.py", line 1255, in request
    self._send_request(method, url, body, headers, encode_chunked)
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/http/client.py", line 1301, in _send_request
    self.endheaders(body, encode_chunked=encode_chunked)
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/http/client.py", line 1250, in endheaders
    self._send_output(message_body, encode_chunked=encode_chunked)
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/http/client.py", line 1010, in _send_output
    self.send(msg)
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/http/client.py", line 950, in send
    self.connect()
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/http/client.py", line 921, in connect
    self.sock = self._create_connection(
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/socket.py", line 843, in create_connection
    raise err
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/socket.py", line 831, in create_connection
    sock.connect(sa)
ConnectionRefusedError: [Errno 111] Connection refused

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/jaap.vandervelde/sandbox/run.py", line 6, in <module>
    r = request.urlopen('http://localhost:8000')
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/urllib/request.py", line 214, in urlopen
    return opener.open(url, data, timeout)
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/urllib/request.py", line 517, in open
    response = self._open(req, data)
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/urllib/request.py", line 534, in _open
    result = self._call_chain(self.handle_open, protocol, protocol +
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/urllib/request.py", line 494, in _call_chain
    result = func(*args)
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/urllib/request.py", line 1371, in http_open
    return self.do_open(http.client.HTTPConnection, req)
  File "/home/jaap.vandervelde/.pyenv/versions/3.9.1/lib/python3.9/urllib/request.py", line 1345, in do_open
    raise URLError(err)
urllib.error.URLError: <urlopen error [Errno 111] Connection refused>

I feel like I'm missing something obvious, but am not seeing it. More so because the exact same script runs with expected results on Windows.

Note that I'm not planning on running anything important with the above code, I just had something like this sit in an old unit test somewhere and cannot figure out why it won't work on this platform.

Answer 1

You'll need to wait. You have no guarantee that the web server has initialized itself and is ready to serve requests by the time you issue your request.