How to make a serial port sniffer sniffing physical port using a python

You should go through pySerial

Only one function can acquire the serial port at a time.

For one-way communication(from machine to PC software), the only way I can think of to sniff from a serial port is to read from a port1 and write to port2, where your machine is writing to port1 and PC software has been modified to read from port2.

import serial

baud_rate = 4800 #whatever baudrate you are listening to
com_port1 = '/dev/tty1' #replace with your first com port path
com_port2 = '/dev/tty2' #replace with your second com port path

listener = serial.Serial(com_port1, baudrate)
forwarder = serial.Serial(com_port2, baudrate)

while 1:
    serial_out = listener.read(size=1)
    print serial_out #or write it to a file 
    forwarder.write(serial_out)

To achieve full duplex(asynchronous two way communication), you need to have a two processes, one for each direction. You will need to synchronize these process in some way. One way to do it could be, while one process reads from port1, the other writes to port2, and vice-versa. Read this question

Why not echo something like:

PC S/W <--> COMn(COM0COM)COMm <--> python monitor & forward <--> COM1 <--> Machine

Software wise you need 2 serial tasks one opens COMm and one opens COM1 and a central logger and anything that comes in on COMm gets logged then forwarded to COM1 and vice verca.