Transcript Last Class - Villanova Department of Computing Sciences
Python for database access and Event driven programming in Python
But first, this week …
• • • • • • • • This is Computer Science Education Week Date chosen to honor Grace Murray Hopper, whose birthday is December 9 Rear Admiral, U.S. Navy Computer Science Pioneer Credited with the term “debugging” Developed the first compiler Conceptualized the idea of a machine independent programming language Explains nano seconds!
Database access in Python
• A set of tutorials, including access to MySQL and PostgreSQL http://python.about.com/od/pythonanddatabases/Connecting_to_Databases_With_Python.htm
• A brief introduction to SQLite3 – Lightweight disk-based database – See http://docs.python.org/library/sqlite3.html
Example, using sqlite3
• First, import sqlite3 • Create an object that represents the database • Use standard sql import sqlite3 • This example from Nakul Rathod def main(): db = sqlite3.connect('test.db') # connect to the database or create the database if it doesn't exist db.row_factory = sqlite3.Row # the powerful Row Factory interface allows one to access information in the database like a dictionary db.execute('drop table if exists test') db.execute('create table test(t1 text, i1 int)’) # the database object accepts standard sql db.execute('insert into test (t1, i1) values(?,?)', ('one', 1)) # you can directly substitute the values in ?,? but variable names allow for code re-usability db.execute('insert into test (t1, i1) values(?,?)', ('two', 2)) db.execute('insert into test (t1, i1) values(?,?)', ('three', 3)) db.commit() # without a commit no changes to the database will be recorded cursor = db.execute('select * from test order by i1') for row in cursor: print row['i1'], row['t1'] # Each record is fully indexed by the field heading due to Row Factory main()
• • • Create a database using sqlite3 Each row in the database contains the following information (Make about 5 entries) – ID (alphanumeric) – Last name (alpha) – First name (alpha) – Hire year (int) Enter data in random order, then display all in order of hire year
Event driven programming
• • This is another way to think about flow control in a program.
An important consideration is parallelism (or pseudo-parallelism) – If there is only one processor, and we write the program to support more than one event at once, then the one processor gives slices of time to each event.
– All modern computers have multi core architecture – more than one processor available. It is possible to have more than one event process happening at the same time.
• • Most of our programs are basically sequential, with excursions into functions and classes for special purposes There is a clearly defined sequence of actions that the program will perform.
Event Driven Programs
• • • In event-driven programs, the sequence of actions is not known in advance The program has the facility to respond to external events and take appropriate action.
Now, programming means – determining what events might happen – recognizing each event – taking appropriate action for the event – waiting for the next • sometimes accommodating simultaneous events
• • An event may be a special case of normal programs – such as an error event We have seen how to handle errors – This is a form of interrupt handling number = 0 while not 1 <= number <= 10: try: number= int(raw_input('Enter number from 1 to 10: ')) if not 1 <= number <= 10: print 'Your number must be from 1 to 10:' except ValueError: print 'That is not a valid integer.'
• • Some programs do nothing except respond to events – Servers listen for a request and respond GUI interfaces present a pallet to the user – The operating system responds to the choices made by the user
Wait loops and threads
• • • One way to implement event handlers is with an endless loop that checks for an indication of the event, then responds If there is only one process running, the event takes control and no other event can be serviced until that one is complete Threads allow multiple flows of execution simultaneously – – Same processor Multiple processors
Book introduces event driven programming in the context of drawing
– Fine, but let’s look at networks •
A brief introduction to networks to start
– Peer – to – Peer – Client-Server
• • • Much of our computing environment is client-server based Server: – provides a service – waits for a request and does something to satisfy the request Client: – The controlling side of the interaction – Connects to a server – Issues a request
• • Server waits for requests Server must be connected to a network Server software waits for requests, does some processing, sends responses
Locating a server
• Web as our primary communication method to servers • URL (or URI) names the server where a resource is provided • Class web page is located at • • • • http://www.csc.villanova.edu/~cassel/8000/Syllabus.html
http: the protocol www.csc.villanova.edu
: the domain rest: the path to the resource Domain Name Server (DNS) translates from the domain name to the IP address of the machine.
– Multi-level – edu, villanova, csc
• • Each machine on the network has a location, called an IP address – think of it like a phone number Many possible connection requests – Within the machine, a port identifies the specific communication line • think of it like an extension in a phone system – A client requests a connection on a particular port to reach the service of interest • Now, messages flow between the client process and the desired server process
• • How does the client request the desired service? A protocol is a standard set of messages to exchange between cooperating entities to achieve a desired result – What protocols do you know?
• • • You know protocols How do you introduce two people?
– Suppose person A is senior to person B Can you think of any other standard message exchanges?
– Anyone remember Lawrence Welk – • And a 1 and a 2 and a 3 …
• • • There are many that we will not consider For Client-server computing – Server listens on the port (end point of the socket) – Client directs a message to the known port – Message content tells the server what service is wanted – Server responds with a standard message, in an agreed format Each side understands only the pre-defined messages!
Python Socket class
s.connect(host, port) s.send(data) s.recv(maxNumChar s) s.close( )
connects socket s to remote host at specified port.
sends data to connected remote host.
receives a maxNumChars from the remote host.
Disconnects socket s from remote host.
• • Get time from the time server at NIST No need to send anything because the server only does one thing. – As soon as the connection is requested, the desired service is identified >>> from socket import socket >>> connection = socket() >>> connection.connect(('time.nist.gov',13)) >>> print connection.recv(1024) 55902 11-12-07 16:41:46 00 0 0 380.0 UTC(NIST) *
Process the returned data
• • We know the exact format of the data, because that is part of the protocol We can now process that, format it, do anything we like with it.
>>> from socket import socket >>> connection = socket() >>> server = 'time.nist.gov' >>> connection.connect((server,13)) >>> fields= connection.recv(1024).split() >>> date = fields >>> >>> time = fields >>> print 'Date (YY-MM-DD) is %s, time is %s (UTC)'%(date,time) Date (YY-MM-DD) is 11-12-07, time is 16:46:50 (UTC) >>>
• • Connect to the NIST time server Print out the current date and time in this format: It is now
There are four possibilities for the outcome of a recv command 1. Receive string of length equal to maximum length specified. May have further data that can be retrieved by another call to recv.
2. Receive less data than maximum. That is the end of that batch of data.
3. No data was sent so recv waits indefinitely.
4. Empty string is received. Connection was closed by sender or network was disrupted.
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• • • Nothing particularly event driven on the client side This is where the event originates The event driven software is on the server side
Writing a server – event driven programming
• • • A server is a basic event-driven program Server – listen on a particular port for incoming connections – create a socket to manage each connection – follow the chosen protocol for communication with the client From the server side – – send means from server to client recv means from client to server
A bit about network protocols
• References to TCP – Transmission control protocol – Connection-oriented protocol for establishing and maintaining a connection between communicating entities on the Internet • handles things like corrupted transmissions, dropped calls, etc.
• • Hides all the details of establishing a reliable connection and making sure all the data is delivered correctly, and in order.
Customize by defining a class to handle each connection
A simple server
• Echo server – Receives a message from a client and sends the same message back.
• Actually useful for testing that a connection is working, and the round trip time for a message # Example from Goldwasser book from SocketServer import TCPServer, BaseRequestHandler class EchoHandler(BaseRequestHandler): def handle(self): message = self.request.recv(1024) self.request.send(message) # may need to customize localhost and port for your machine echoServer = TCPServer( ('localhost', 9128), EchoHandler) echoServer.serve_forever()
The client side of echo
• • • Connect to the server Send the message Get the message back >>> from socket import socket >>> echo = socket() >>> echo.connect(('localhost',9128)) >>> echo.send('This is a test') 14 >>> print echo.recv(1024) This is a test >>>
• • Get the echo server running on your machine.
Make the echo client connect to the server and echo a message.
Basic web server
• • Similar to the echo server, except return a requested file instead of echoing the message Overall outline – Accept the connection – Receive the file request – Search for the file in the path provided – If not there, return error message (404!) – If there, return the file
from SocketServer import TCPServer, BaseRequestHandler Note: runs on local host, not accessible over the Internet class WebHandler(BaseRequestHandler): def handle(self): command = self.request.recv(1024) if command[:3] == 'GET': pagename = command.split()[1:] # remove leading '/' for filename try: requestedFile = file(pagename, 'r') content = requestedFile.read() requestedFile.close() header = 'HTTP/1.0 200 OK\r\n' header += 'Content-Length: %d\r\n\r\n' % len(content) self.request.send(header) Very simple. Assume file in local directory Note creation of file self.request.send(content) except IOError: # could not open the file header information sent back to self.request.send('HTTP/1.0 404 Not Found\r\n\r\n') requestor with the file webServer = TCPServer( ('localhost', 8080), WebHandler) webServer.serve_forever()
• • If not now, then later – Write the corresponding client to request a file and receive it back.
• • • • So far, connection made, something happens, connection ends.
Server is then free to handle another connection.
Client is disconnected entirely Some applications require persistence – continuing connection between client and server
Case Study: Network Chat-Room
• • • • • Develop our own protocol for chat room.
Connection has permanence unlike previous examples – persistent connection.
Client must monitor keyboard and also listen to the socket. Use multithreading to accomplish this. Without multithreading can get hung-up on listening to socket or the keyboard.
Following two slides show different messages sent from client to server and server to client.
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Client to Server Messages
Message Type Format
Join the room using given identity.
'ADD %s\n' % screenName 'MESSAGE %s\n'%content Broadcast the message to everyone.
Send a private message.
'PRIVATE %s\n%s\n' %(recipient, content) Quit the chat room.
'QUIT\n' Terry Scott University of Northern Colorado 2007 Prentice Hall 37
Server to Client Messages
Message Type Format
New user has joined. 'NEW %s\n' % screenname Message was broadcast to everyone.
Private message was sent to user.
'MESSAGE %s\n%s\n' % (sender, content) 'PRIVATE %s\n%s\n' % (sender, content) Someone has left the room.
'LEFT %s\n' % screenName Acknowleges 'GOODBYE\n' request to quit.
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• • • • Uses ThreadingTCPServer rather than TCPServer. Needed since can be multiple people in the chat room.
_broadcast function used to send to clients in the chatroom.
Each person that joins the chat-room is given a new socket that is accessed via a dictionary with the person screen name as the key.
Code for the server is on the next three slides.
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Chat Server Code
from SocketServer import ThreadingTCPServer, BaseRequestHandler _socketLookup = dict() def _broadcast(announcement): for connection in _socketLookup.values(): connection.send(announcement) class ChatHandler(BaseRequestHandler): def handle(self): username = 'Unknown' active = True Terry Scott University of Northern Colorado 2007 Prentice Hall 40
Chat Server Code (continued)
while active: transmission = self.request.recv(1024) if transmission: command = transmission.split() data = transmission[1+len(command):] if command == 'ADD': username = data.strip() _socketLookup[username] = self.request
_broadcast('NEW %s\n' % username) elif command == 'MESSAGE': _broadcast('MESSAGE %s\n%s\n' % (username,data)) Terry Scott University of Northern Colorado 2007 Prentice Hall 41
Chat Server Code (continued)
elif command == 'PRIVATE': rcpt = data.split('\n') if rcpt in _socketLookup: content = data.split('\n') _socketLookup[rcpt].send( 'PRIVATE %s\n%s\n' % (username,content)) elif command == 'QUIT': active = FALSE self.request.send('GOODBYE\n')
active = False self.request.close() _socketLookup.pop(username) _broadcast('LEFT %s\n' %username) myServer=ThreadingTCPServer('localhost',9000), ChatHandler) myServer.serve_forever() Terry Scott University of Northern Colorado 2007 Prentice Hall 42
Sufficient unto the day
• • • • • or the semester Hopefully, we have accomplished a good bit in the last four months Python, object-oriented programming, Some natural language processing – as examples of language modules and algorithms A bit of networking even.