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Python for database access and Event driven programming in Python

Python for database access and Event driven programming in Python. 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

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Python for database access and Event driven programming in Python

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  1. Python for database access and Event driven programming in Python

  2. 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 • NOTE: There are many details of database access. Here we will do a brief example and establish the ability to do the basic operations. See this website for a complete list of features

  3. SQL3litehttp://docs.python.org/2/library/sqlite3.html import sqlite3 conn = sqlite3.connect('sample.db') c=conn.cursor() #Create table c.execute('''CREATE TABLE stocks (date text, trans text, symbol text, \ qty real, price real)''') #Insert a row of data c.execute("INSERT INTO stocks VALUES \ ('2006-01-05','BUY','RHAT',100,35.14)") #Save (commit) the changes conn.commit() #Close the connection. #Be sure the changes are committed first. conn.close() New database (sample.db) created with one row of data

  4. Access the data import sqlite3 cnx = sqlite3.connect('sample.db') c = cnx.cursor() t = ('RHAT',) c.execute('SELECT * FROM stocks WHERE symbol=?', t) print c.fetchone() print '======================\n' # Larger example that inserts many records at a time purchases = [('2006-03-28','BUY', 'IBM', 1000, 45.00), ('2006-04-05','BUY', 'MSFT', 1000, 72.00), ('2006-04-06','SELL', 'IBM', 500, 53.00), ] c.executemany('INSERT INTO stocks VALUES (?,?,?,?,?)', purchases) for row in c.execute('SELECT * FROM stocks ORDER BY \ price'): print row

  5. Output (u'2006-01-05', u'BUY', u'RHAT', 100.0, 35.14) ====================== (u'2006-01-05', u'BUY', u'RHAT', 100.0, 35.14) (u'2006-03-28', u'BUY', u'IBM', 1000.0, 45.0) (u'2006-04-06', u'SELL', u'IBM', 500.0, 53.0) (u'2006-04-05', u'BUY', u'MSFT', 1000.0, 72.0) The first entry in the last set came from the original values in the db, from the program that created the db.

  6. Another sqllite3 example • First, import sqlite3 • Create an object that represents the database • Use standard sql • This example from NakulRathod import sqlite3 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 db.execute('drop table if exists test') db.execute('create table test(t1 text, i1 int)') 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()

  7. Spot check • 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

  8. 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.

  9. So far • 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.

  10. 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

  11. Error 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:' exceptValueError: print 'That is not a valid integer.'

  12. Servers, GUIs • 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

  13. 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

  14. Network programming • 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

  15. Client-Server Computing • 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

  16. Server must be connected to a network • Server software waits for requests, does some processing, sends responses Server waits for requests

  17. 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/~cassel8000/2012/8000Syllabus.html • 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

  18. Network connections • 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

  19. Protocols • 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?

  20. Spot check • You know protocols • How do you introduce two people? • Suppose person A is senior to person B • How do you answer a phone call • In a business context? • If you know the person who is calling? • How do you issue a formal invitation? • How do you respond to a formal invitation? • Can you think of any other standard message exchanges? • Anyone remember Lawrence Welk – • And a 1 and a 2 and a 3 …

  21. Network protocols • 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!

  22. Threads • Multiple threads are necessary when a program must appear to be doing more than one sequence of steps at the same time. • Example: Waiting for more than one kind of event to occur. Have the code that waits for each in a separate thread.

  23. Thread example from threading import Thread from time import sleep class MyThread(Thread): def run(self): print 'Second thread begins.' sleep(5) print 'Second thread is done.’ secondThread=MyThread() secondThread.start() print 'The program is done.' Second thread begins. The program is done. Second thread is done.

  24. Python Socket class

  25. Simple client • 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) Output: 56259 12-11-28 19:30:19 00 0 0 118.4 UTC(NIST) *

  26. 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() connection.connect(('time.nist.gov',13)) fields=connection.recv(1024).split() date = fields[1] time = fields[2] print 'Date (YY-MM-DD) is %s, time is %s (UTC)’\ %(date,time)

  27. Spot check • Connect to the NIST time server • Print out the current date and time in this format: It is now <time> on <date> Note the time reported. Is it right? How would you adjust if needed? • How would you make the date be in the form • November <day>, 2012 • (or, if you prefer) <day> November 2012

  28. recv Method • There are four possibilities for the outcome of a recv command • Receive string of length equal to maximum length specified. May have further data that can be retrieved by another call to recv. • Receive less data than maximum. That is the end of that batch of data. • No data was sent so recv waits indefinitely. • Empty string is received. Connection was closed by sender or network was disrupted. Terry Scott University of Northern Colorado 2007 Prentice Hall

  29. Client side • Nothing particularly event driven on the client side • This is where the event originates • The event driven software is on the server side

  30. 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

  31. 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.

  32. TCPServer class • 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

  33. 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()

  34. 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)) msg = raw_input("Enter a message to echo:") echo.send(msg) print echo.recv(1024)

  35. Spot check • Get the echo server running on your machine. • Make the echo client connect to the server and echo a message.

  36. 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

  37. Web server Note: runs on local host, not accessible over the Internet from SocketServer import TCPServer, BaseRequestHandler class WebHandler(BaseRequestHandler): def handle(self): command = self.request.recv(1024) if command[:3] == 'GET': pagename = command.split()[1][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) self.request.send(content) except IOError: # could not open the file self.request.send('HTTP/1.0 404 Not Found\r\n\r\n') webServer = TCPServer( ('localhost', 8080), WebHandler) webServer.serve_forever() Very simple. Assume file in local directory Note creation of file header information sent back to requestor with the file

  38. Spot check • With a partner, step through the web server code and be sure you understand each line. Take turns explaining the lines to each other, one line at a time.

  39. Persistence • 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

  40. 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. Terry Scott University of Northern Colorado 2007 Prentice Hall

  41. Client to Server Messages Terry Scott University of Northern Colorado 2007 Prentice Hall

  42. Server to Client Messages Terry Scott University of Northern Colorado 2007 Prentice Hall

  43. Chat Server • 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 who joins the chat-room is given a new socket, which is accessed via a dictionary with the person’s screen name as the key. • Code for the server is on the next three slides. Terry Scott University of Northern Colorado 2007 Prentice Hall

  44. Last login: Wed Dec 5 17:32:20 on ttys003 -bash: /Users/lcassel/.bash_profile: line 10: unexpected EOF while looking for matching `"' -bash: /Users/lcassel/.bash_profile: line 12: syntax error: unexpected end of file BootsMac:~ lcassel$ cd pythonwork/pythonexamples BootsMac:pythonexampleslcassel$ ls chatclient.pychatserver.pythread.py BootsMac:pythonexampleslcassel$ vi chatserver.py 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 while active: transmission = self.request.recv(1024) if transmission: command = transmission.split()[0] data = transmission[1+len(command):] Chat Server Code Terry Scott University of Northern Colorado 2007 Prentice Hall

  45. 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)) elif command == 'PRIVATE': rcpt = data.split('\n')[0] if rcpt in _socketLookup: content = data.split('\n')[1] _socketLookup[rcpt].send('PRIVATE %s\n%s\n' \ %(username,content)) elif command == 'quit': active = False self.request.send('GOODBYE\n') else: active = False self.request.close() _socketLookup.pop(username) _broadcast('LEFT %s\n' %username) myServer = ThreadingTCPServer(('localhost',9000), ChatHandler) myServer.serve_forever()

  46. Last login: Wed Dec 5 17:50:02 on ttys004 -bash: /Users/lcassel/.bash_profile: line 10: unexpected EOF while looking for matching `"' -bash: /Users/lcassel/.bash_profile: line 12: syntax error: unexpected end of file BootsMac:~ lcassel$ cd pythonwork/pythonexamples BootsMac:pythonexampleslcassel$ vi chatclient.py # Program: chatclient.py # Authors: Michael H. Goldwasser # David Letscher # # This example is discussed in Chapter 16 of the book # Object-Oriented Programming in Python # from socket import socket from threading import Thread class IncomingThread(Thread): def run(self): stillChatting = True while stillChatting: # wait for more incoming data transmission = server.recv(1024) # 'server' will be # defined globally at line 27 lines = transmission.split('\n')[:-1] i = 0 Chat client

  47. while i < len(lines): command = lines[i].split()[0] # first keyword param = lines[i][len(command)+1: ] # remaining info if command == 'GOODBYE': stillChatting = False elif command == 'NEW': print '==>', param, 'has joined the chat room' elif command == 'LEFT': print '==>', param, 'has left the chat room' elif command == 'MESSAGE': i += 1 # need next line for content print '==>', param + ': ' + lines[i] elif command == 'PRIVATE': i += 1 # need next line for content print '==>', param + ' [private]: ' + lines[i] i += 1

  48. instructions = """ -------------------------------------------- Welcome to the chat room. To quit, use syntax, quit To send private message to 'Joe' use syntax, private Joe:how are you? To send message to everyone, use syntax, hi everyone! -------------------------------------------- """ server = socket() # shared by both threads server.connect( ('localhost', 9000) ) # could be a remote host username = raw_input('What is your name: ').strip() server.send('ADD %s\n' % username ) incoming = IncomingThread() incoming.start()

  49. print instructions active = True # main thread for user input while active: message = raw_input() # wait for more user input if message.strip(): if message.rstrip().lower() == 'quit': server.send('QUIT\n') active = False elifmessage.split()[0].lower() == 'private': colon = message.index(':') friend = message[7:colon].strip() server.send('PRIVATE %s\n%s\n' \ % (friend,message[1+colon: ]) ) else: server.send('MESSAGE ' + message)

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