Handling socket errors gracefully

In any networking application, it is very common that one end is trying to connect, but the other party is not responding due to networking media failure or any other reason. The Python socket library has an elegant method of handing these errors via the socket.error exceptions. In this recipe, a few examples are presented.

How to do it...

Let us create a few try-except code blocks and put one potential error type in each block. In order to get a user input, the argparse module can be used. This module is more powerful than simply parsing command-line arguments using sys.argv. In the try-except blocks, put typical socket operations, for example, create a socket object, connect to a server, send data, and wait for a reply.

The following recipe illustrates the concepts in a few lines of code.

Listing 1.7 shows socket_errors as follows:

#!/usr/bin/env python 
# Python Network Programming Cookbook, Second Edition -- Chapter - 1 
# This program is optimized for Python 2.7.12 and Python 3.5.2. 
# It may run on any other version with/without modifications. 
 
import sys 
import socket 
import argparse  
 
 
def main(): 
    # setup argument parsing 
    parser = argparse.ArgumentParser(description='Socket Error Examples') 
    parser.add_argument('--host', action="store", dest="host",
                                                        required=False) 
    parser.add_argument('--port', action="store", dest="port", type=int,
                                                        required=False) 
    parser.add_argument('--file', action="store", dest="file",
                                                        required=False) 
    given_args = parser.parse_args() 
    host = given_args.host 
    port = given_args.port 
    filename = given_args.file 
     
    # First try-except block -- create socket  
    try: 
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) 
    except socket.error as e: 
        print ("Error creating socket: %s" % e) 
        sys.exit(1) 
     
    # Second try-except block -- connect to given host/port 
    try: 
        s.connect((host, port)) 
    except socket.gaierror as e: 
        print ("Address-related error connecting to 
                server: %s" % e) 
        sys.exit(1) 
    except socket.error as e: 
        print ("Connection error: %s" % e) 
        sys.exit(1) 
     
    # Third try-except block -- sending data 
    try: 
        msg = "GET %s HTTP/1.0\r\n\r\n" % filename 
        s.sendall(msg.encode('utf-8')) 
    except socket.error as e: 
        print ("Error sending data: %s" % e) 
        sys.exit(1) 
     
    while 1: 
        # Fourth tr-except block -- waiting
           to receive 
          data from remote host 
        try: 
            buf = s.recv(2048) 
        except socket.error as e: 
            print ("Error receiving data: %s" % e) 
            sys.exit(1) 
        if not len(buf): 
            break 
        # write the received data 
        sys.stdout.write(buf.decode('utf-8'))  
     
if __name__ == '__main__': 
    main() 

How it works...

In Python, passing command-line arguments to a script and parsing them in the script can be done using the argparse module. This is available in Python 2.7. For earlier versions of Python, this module is available separately in Python Package Index (PyPI). You can install this via easy_install or pip.

In this recipe, three arguments are set up—a hostname, port number, and filename. The usage of this script is as follows:

$ python 1_7_socket_errors.py --host=<HOST> 
  --port=<PORT> --file=<FILE>
    
In the preceding recipe, msg.encode('utf-8') 
encodes the message into UTF-8, and 
buf.decode('utf-8') decodes the received UTF-8 
format.
    
  

If you try the preceding recipe with a non-existent host, this script will print an address error as follows:

$ python 1_7_socket_errors.py 
  --host=www.pytgo.org --port=8080 
  --file=1_7_socket_errors.py 
Address-related error connecting to 
server: [Errno -2] Name or service not known
  

If there is no service on a specific port and if you try to connect to that port, then this will throw a connection timeout error as follows:

$ python 1_7_socket_errors.py 
  --host=www.python.org --port=8080 
  --file=1_7_socket_errors.py 
  

This will return the following error since the host, www.python.org, is not listening on port 8080:

Connection error: [Errno 110] Connection timed out
  

However, if you send an arbitrary request as a correct request to a correct port, the error may not be caught at the application level. For example, running the following script returns no error, but the HTML output tells us what's wrong with this script:

$ python 1_7_socket_errors.py 
  --host=www.python.org --port=80 
  --file=1_7_socket_errors.py
    
HTTP/1.1 500 Domain Not Found
Server: Varnish
Retry-After: 0
content-type: text/html
Cache-Control: private, no-cache
connection: keep-alive
Content-Length: 179
Accept-Ranges: bytes
Date: Thu, 01 Jun 2017 22:02:24 GMT
Via: 1.1 varnish
Connection: close
    
    
<html>
<head>
<title>Fastly error: unknown domain </title>
</head>
<body>
Fastly error: unknown domain: . Please check that this domain has been added to a service.</body></html>
    
  

In the preceding example, four try-except blocks have been used. All blocks use socket.error except for the second block, which uses socket.gaierror. This is used for address-related errors. There are two other types of exceptions—socket.herror is used for legacy C API, and if you use the settimeout() method in a socket, socket.timeout will be raised when a timeout occurs on that socket.