Use non privileged tcp port in examples (#109)

The server example had port 502 hard coded. This port is a privileged port.
Trying to use it without permissions could result in a failure of the example.
Port 502 is still the default, but a nice error message is printed to warn the user
about the problem. Also a flag has been introduced to change the port.

To keep the example client working with the example server code a similar
change has been made to the example client.

README.rst

@@ -36,6 +36,7 @@ Creating a Modbus TCP server is easy:
     import logging
     from socketserver import TCPServer
     from collections import defaultdict
+    from argparse import ArgumentParser
 
     from umodbus import conf
     from umodbus.server.tcp import RequestHandler, get_server
@@ -44,23 +45,37 @@ Creating a Modbus TCP server is easy:
     # Add stream handler to logger 'uModbus'.
     log_to_stream(level=logging.DEBUG)
 
-    # A very simple data store which maps addresss against their values.
+    # A very simple data store which maps addresses against their values.
     data_store = defaultdict(int)
 
     # Enable values to be signed (default is False).
     conf.SIGNED_VALUES = True
 
-    TCPServer.allow_reuse_address = True
-    app = get_server(TCPServer, ('localhost', 502), RequestHandler)
+    # Parse command line arguments
+    parser = ArgumentParser()
+    parser.add_argument("-b", "--bind", default="localhost:502")
 
+    args = parser.parse_args()
+    if ":" not in args.bind:
+        args.bind += ":502"
+    host, port = args.bind.rsplit(":", 1)
+    port = int(port)
 
-    @app.route(slave_ids=[1], function_codes=[3, 4], addresses=list(range(0, 10)))
+    TCPServer.allow_reuse_address = True
+    try:
+        app = get_server(TCPServer, (host, port), RequestHandler)
+    except PermissionError:
+        print("You don't have permission to bind on {}".format(args.bind))
+        print("Hint: try with a different port (ex: --bind localhost:50200)")
+        exit(1)
+
+    @app.route(slave_ids=[1], function_codes=[1, 2], addresses=list(range(0, 10)))
     def read_data_store(slave_id, function_code, address):
         """" Return value of address. """
         return data_store[address]
 
 
-    @app.route(slave_ids=[1], function_codes=[6, 16], addresses=list(range(0, 10)))
+    @app.route(slave_ids=[1], function_codes=[5, 15], addresses=list(range(0, 10)))
     def write_data_store(slave_id, function_code, address, value):
         """" Set value for address. """
         data_store[address] = value
@@ -82,26 +97,34 @@ Doing a Modbus request requires even less code:
 
     #!/usr/bin/env python
     # scripts/examples/simple_tcp_client.py
-    import socket
+    from argparse import ArgumentParser
+    from socket import create_connection
 
     from umodbus import conf
     from umodbus.client import tcp
 
     # Enable values to be signed (default is False).
     conf.SIGNED_VALUES = True
 
-    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
-    sock.connect(('localhost', 502))
+    # Parse command line arguments
+    parser = ArgumentParser()
+    parser.add_argument("-a", "--address", default="localhost:502")
+
+    args = parser.parse_args()
+    if ":" not in args.address:
+        args.address += ":502"
+    host, port = args.address.rsplit(":", 1)
+    port = int(port)
 
     # Returns a message or Application Data Unit (ADU) specific for doing
     # Modbus TCP/IP.
     message = tcp.write_multiple_coils(slave_id=1, starting_address=1, values=[1, 0, 1, 1])
 
-    # Response depends on Modbus function code. This particular returns the
-    # amount of coils written, in this case it is.
-    response = tcp.send_message(message, sock)
+    with create_connection((host, port)) as sock:
+        # Response depends on Modbus function code. This particular returns the
+        # amount of coils written, in this case it is.
+        response = tcp.send_message(message, sock)
 
-    sock.close()
 
 Features
 --------

scripts/examples/simple_tcp_client.py

@@ -1,22 +1,29 @@
 #!/usr/bin/env python
 # scripts/examples/simple_tcp_client.py
-import socket
+from argparse import ArgumentParser
+from socket import create_connection
 
 from umodbus import conf
 from umodbus.client import tcp
 
 # Enable values to be signed (default is False).
 conf.SIGNED_VALUES = True
 
-sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
-sock.connect(('localhost', 502))
+# Parse command line arguments
+parser = ArgumentParser()
+parser.add_argument("-a", "--address", default="localhost:502")
+
+args = parser.parse_args()
+if ":" not in args.address:
+    args.address += ":502"
+host, port = args.address.rsplit(":", 1)
+port = int(port)
 
 # Returns a message or Application Data Unit (ADU) specific for doing
 # Modbus TCP/IP.
 message = tcp.write_multiple_coils(slave_id=1, starting_address=1, values=[1, 0, 1, 1])
 
-# Response depends on Modbus function code. This particular returns the
-# amount of coils written, in this case it is.
-response = tcp.send_message(message, sock)
-
-sock.close()
+with create_connection((host, port)) as sock:
+    # Response depends on Modbus function code. This particular returns the
+    # amount of coils written, in this case it is.
+    response = tcp.send_message(message, sock)

scripts/examples/simple_tcp_server.py

@@ -1,8 +1,9 @@
 #!/usr/bin/env python
-# scripts/examples/simple_data_store.py
+# scripts/examples/simple_tcp_server.py
 import logging
 from socketserver import TCPServer
 from collections import defaultdict
+from argparse import ArgumentParser
 
 from umodbus import conf
 from umodbus.server.tcp import RequestHandler, get_server
@@ -17,8 +18,23 @@
 # Enable values to be signed (default is False).
 conf.SIGNED_VALUES = True
 
+# Parse command line arguments
+parser = ArgumentParser()
+parser.add_argument("-b", "--bind", default="localhost:502")
+
+args = parser.parse_args()
+if ":" not in args.bind:
+    args.bind += ":502"
+host, port = args.bind.rsplit(":", 1)
+port = int(port)
+
 TCPServer.allow_reuse_address = True
-app = get_server(TCPServer, ('localhost', 502), RequestHandler)
+try:
+    app = get_server(TCPServer, (host, port), RequestHandler)
+except PermissionError:
+    print("You don't have permission to bind on {}".format(args.bind))
+    print("Hint: try with a different port (ex: --bind localhost:50200)")
+    exit(1)
 
 
 @app.route(slave_ids=[1], function_codes=[1, 2], addresses=list(range(0, 10)))
@@ -32,6 +48,7 @@ def write_data_store(slave_id, function_code, address, value):
     """" Set value for address. """
     data_store[address] = value
 
+
 if __name__ == '__main__':
     try:
         app.serve_forever()