Popen.stdout.readline() works in Python 3.2 but returns empty string in Python 3.6

Question

I am using Popen to start a telnet process over ssh, send commands through the telnet and check the process for output to monitor the state of the telnet. The weird thing I encountered is that the code works fine with Python 3.2.3 but when I run it in Python 3.6.5 with no code changes, it fails to get the output.

I have tried

flushing the stdout
waiting up to 10s after stdout.write
checked stderr

def nonblockingread(sout): fd = output.fileno() fl = fcntl.fcntl(fd, fcntl.F_GETFL) try: return sout.read() except: return "" process = Popen(shlex.split("ssh anon@server \"telnet 0 2323\""), stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE) print(nonblockingread(process.stdout)) # works for both versions process.stdin.write(b"start service 4\n") print(nonblockingread(process.stdout)) # works in 3.2 but not 3.6 (prints empty line) print(process.stdout.readline()) # also prints empty line in 3.6

Why are you writing to the readable process.stdout rather than the writable process.stdin? — Dan D.
– Dan D., Commented Apr 24, 2019 at 23:26
There seems to be some code missing—you never set anything non-blocking, for instance. Is your “service 4” actually started? — Davis Herring
– Davis Herring, Commented Apr 25, 2019 at 0:23

Davis Herring · Accepted Answer · 2019-04-25 00:28:54Z

Buffering was turned on by default in 3.2.4 and 3.3.1, having been turned off inadvertently in Python 3. You need to flush your write to process.stdin for the other side to see anything.

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