Last Class - Villanova Department of Computing Sciences

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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
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•
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First, import sqlite3
Create an object that represents the database
Use standard sql
This example from Nakul Rathod
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
# 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()
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
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.
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.
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
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:'
except ValueError:
print 'That is not a valid integer.'
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
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
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
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
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
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
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?
Spot check
• 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 …
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!
Python Socket class
Syntax
Semantics
s.connect(host, port) connects socket s to
remote host at specified
port.
s.send(data)
sends data to connected
remote host.
s.recv(maxNumChar receives a
s)
maxNumChars from the
remote host.
s.close( )
Disconnects socket s
from remote host.
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)
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[1]
>>>
>>> time = fields[2]
>>> 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)
>>>
Spot check
• Connect to the NIST time server
• Print out the current date and time in
this format:
It is now <time> on <date>
• How would you make the date be in the
form
– December <day>, 2011
– (or, if you prefer) <day> December 2011
recv Method
•
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.
Terry Scott University of Northern Colorado
2007 Prentice Hall
24
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
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.
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
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
>>>
Spot check
• 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
Web server
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][1:]
# remove leading '/' for filename
try:
requestedFile = file(pagename, 'r')
Very simple.
content = requestedFile.read()
Assume file in local
requestedFile.close()
directory
header = 'HTTP/1.0 200 OK\r\n'
header += 'Content-Length: %d\r\n\r\n' % len(content)
self.request.send(header)
Note creation of file
self.request.send(content)
header information
except IOError:
# could not open the file sent back to
self.request.send('HTTP/1.0 404 Not Found\r\n\r\n')
requestor with the
webServer = TCPServer( ('localhost', 8080), WebHandler)
webServer.serve_forever()
file
Spot check
• If not now, then later –
• Write the corresponding client to
request a file and receive it back.
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
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
36
Client to Server Messages
Message Type
Join the room
using given
identity.
Broadcast the
message to
everyone.
Send a private
message.
Format
'ADD %s\n' % screenName
'MESSAGE %s\n'%content
'PRIVATE %s\n%s\n'
%(recipient, content)
Quit the chat
'QUIT\n'
Terry Scott University of Northern Colorado
room.
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 'LEFT %s\n' % screenName
room.
Acknowleges
'GOODBYE\n'
request to quit.
Terry Scott University of Northern Colorado
2007 Prentice Hall
38
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 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.
Terry Scott University of Northern Colorado
2007 Prentice Hall
39
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()[0]
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')[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()
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.

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