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I am trying to readlines from the tcp server that I ran in the same script. I am able to send one command and reads its output but at the end (after reading all outputs) it looks like that program hangs in readline, I have tried almost all the solutions here and here but still it hangs.

Most of these solutions propose to check if output of readline is none or not but in my case program never returns from last read and just hangs in there.

tcp server is not in my control, or say I just have to test server script therefore I can not modify it. Also, is it possible to send commands to runing server using python without using subprocess? any better alternative?

def subprocess_cmd(command):
   process=subprocess.Popen(command,stdin=subprocess.PIPE,stderr=subprocess.STDOUT,stdout=subprocess.PIPE, shell=True)

for cmd in ['python3 -u tcp_server.py 123 port1']:
    subprocess_cmd(cmd)

process.stdin.write('command like print_list')
process.stdin.flush()
while True:
  line=process.stdout.readline()
  if line == '':
        break
Neuron
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CryptoKitty
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1 Answers1

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readline hangs because your TCP connection is still open and readline expects more data to come in. You must close the connection from server side to notify readline that there is nothing more to read. Usually it is done by closing socket on client side notifying the server that there will not be any more requests to it. When server finishes processing all your commands it closes socket too. And this is the signal for you that you have received all the data that server sent to you.

Or, alternatively, if you don't want to close the connection, you must invent delimiters which will mark end of response. So the client will stop calling readline when such delimiter is read.

Alexey Guseynov
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  • you are right I also have same feeling about it. But please read my edit and let me know if anything I can do ? – CryptoKitty Dec 04 '16 at 21:52
  • According to your code, tcp_server.py is not a TCP server. At least you communicate with it using standard input/output streams, not TCP. Anyway, the reason is the same: you are not closing stdin. And the best way to do it is to use `subprocess.communicate`. Reason is described in documentation to the function. – Alexey Guseynov Dec 04 '16 at 21:59