is_open doesn't work

[VictoriaPresnyakova]

Hi!
is_open doesn't return the correct status when I check connection without trying to read or write

[AnderssonDavid]

I have noticed this as well. I just poll a random register within a "try/except" block. After a poll, the "is_open" does work for me.

[sourceperl]

Hi, in the ModbusClient code, the is_open property depends on the state of the Python socket like this :

    @property
    def is_open(self):
        """Get current status of the TCP connection (True = open)."""
        return self._sock.fileno() > 0

From my testing, the socket state is updated only when we call a send() or recv() on that socket. So yes, you need to try a read/write to update the socket state.

[charliehuddleston]

I just verified the following works, is this how to reproduce the issue?

code1.py or in a terminal window:

from pyModbusTCP.server import ModbusServer, DataBank
mbdb = DataBank()
s = ModbusServer(host="0.0.0.0", port=2502, data_bank=mbdb)
s.start()

code2.py or in a second terminal window:

from pyModbusTCP.client import ModbusClient
c = ModbusClient(host="127.0.0.1", port=2502, auto_open=False, auto_close=False)
c.open()

c.is_open shows True without performing any read/write operations.

[sourceperl]

Unfortunately, the operation of the socket module is not trivial (more info on https://docs.python.org/3/library/socket.html). To try to be clear, here is a small program that details in four steps, the normal operation of pyModbusTCP. It itself depends on the operation of the standard socket module of python

from pyModbusTCP.client import ModbusClient
from pyModbusTCP.server import ModbusServer

# init server
s = ModbusServer(port=5020, no_block=True)
s.start()

# init client
c = ModbusClient(port=5020, auto_open=False, auto_close=False)

# this should return -1 for fileno and is_open = False
print(f'step 1: fileno = {c._sock.fileno()}  is_open = {c.is_open}')

# open TCP socket (call c._sock.connect() and exchange 3 packets with server -> update socket status)
c.open()

# this should return a postive value for fileno and is_open = True
print(f'step 2: fileno = {c._sock.fileno()}  is_open = {c.is_open}')

# close socket on server part
s.stop()

# this should return a postive value for fileno and is_open = True
print(f'step 3: fileno = {c._sock.fileno()}  is_open = {c.is_open}')

# immediately return a None value (under the hood this call _sock.send and _sock.recv -> update socket status)
c.read_coils(0)

# this should return -1 for fileno and is_open = False
print(f'step 4: fileno = {c._sock.fileno()}  is_open = {c.is_open}')