1. Python Gotya’s

be careful to ensure that your code is indented correctly
be very careful not to name your file to a name used by a library you are importing
for example do not call this file string.py

#!/usr/bin/python

import string


words=string.split(’hello world again’)
print words

the python interpreter will read your file twice
- one when you run the file
- and again when it comes across the import string !
name the file teststring and it will work fine
- if you did call it string.py and run then you will need to remove string.py and also string.pyc

2. Python and file handling

file manipulation primitives are by default available
- no need to import library to, read, write files
creating a simple text file

#!/usr/bin/python


file = open(’newfile.txt’, ’w’)
file.write(’hello world in the new file\n’)
file.write(’and another line\n’)
file.close()

#!/usr/bin/python


file = open(’newfile.txt’, ’w’)
file.writelines([’hello world in the new file\n’,
                 ’and another line\n’])
file.close()

#!/usr/bin/python


file = open(’newfile.txt’, ’r’)
for line in file.readlines():
    print line

many ways to read a file
- file.read() returns a string containing all characters in the file
- file.read(N) returns a string containing next N characters
- file.readline() returns a string containing characters up to \n
- file.readlines() returns the complete file as a list of strings each separated by \n

3. Further Python Networking

many python modules which give access to application layer networking services
- ftp, http, telnet, etc
here is how the http protocol utilized in Python:

#!/usr/bin/python

import httplib


h = httplib.HTTP(’floppsie.comp.glam.ac.uk’)
h.putrequest(’GET’, ’/index.html’)
h.putheader(’Accept’, ’text/html’)
h.putheader(’Accept’, ’text/plain’)
h.endheaders()
errcode, errmsg, headers = h.getreply()
print "return value is", errcode # Should be 200
f = h.getfile()
data = f.read()
f.close()
print data

the httplib provides access to the client side http protocol

#!/usr/bin/python

import httplib, urllib


params = urllib.urlencode({’spam’: 1, ’eggs’: 2,
                           ’bacon’: 0})
h = httplib.HTTP("www.musi-cal.com:80")
h.putrequest("POST", "/cgi-bin/query")
h.putheader("Content-length", "%d" % len(params))
h.putheader(’Accept’, ’text/plain’)
h.putheader(’Host’, ’www.musi-cal.com’)
h.endheaders()
h.send(paramstring)
reply, msg, hdrs = h.getreply()
print errcode # should be 200
data = h.getfile().read() # get the raw HTML

sometimes you may have to implement your own application layer protocol
in which case you use sockets (a transport layer service)

#!/usr/bin/python

import sys
from socket import *
serverHost = ’localhost’
serverPort = 2000

# create a TCP socket
s = socket(AF_INET, SOCK_STREAM)


s.connect((serverHost, serverPort))
s.send(’Hello world’)
data = s.recv(1024)
print data

#!/usr/bin/python

from socket import *
myHost = ’’
myPort = 2000

# create a TCP socket
s = socket(AF_INET, SOCK_STREAM)
# bind it to the server port number
s.bind((myHost, myPort))
# allow 5 pending connections
s.listen(5)


while 1:
    # wait for next client to connect
    connection, address = s.accept()
    while 1:
        data = connection.recv(1024)
        if data:
            connection.send(’echo -> ’ + data)
        else:
            break
    connection.close()

4. IMAP library

#!/usr/bin/python

import getpass, imaplib, string


m = imaplib.IMAP4_SSL(’moppsy.comp.glam.ac.uk’)
m.login(getpass.getuser(), getpass.getpass())
m.select()
typ, data = m.search(None, ’ALL’)
print "typ = ", typ
print "data = ", data
for num in string.split(data[0]):
    typ, data = m.fetch(num, ’(RFC822)’)
    print ’Message %s\n%s\n’ % (num, data[0][1])
m.logout()

tutorial read the Python documentation and modify the imap example above to delete your read email messages

5. moppsy

is a Debian GNU/Linux client and server and it will run an imap service
- you can ssh into moppsy.comp.glam.ac.uk and you can email yourself a test message

ssh moppsy.comp.glam.ac.uk
Login: u012345678
Password:
$ mail -s "this is a test" 012345678@localhost
this is the body of the message
^D
$ exit

6. Tutorial

command line arguments familiarity, copy this program and call it, arguments.py

#!/usr/bin/python

import sys, getopt

def Usage ():
    print "arguments.py [-v][-p][-h]"

try:
    optlist, list = getopt.getopt(sys.argv[1:],
                                  ’:vph’)
except getopt.GetoptError:
    Usage()
    print "incorrect argument given"
    sys.exit(1)


for opt in optlist:
    if opt[0] == ’-h’:
        Usage()
    if opt[0] == ’-v’:
        print "verbose found"
    if opt[0] == ’-p’:
        print "probeonly found"

run this program using a terminal and try the following commands:

python arguments.py -v
python arguments.py -h
python arguments.py -p
python arguments.py -p -v
python arguments.py -pv
python arguments.py -pvh
python arguments.py -pv -h
python arguments.py -vp -h
python arguments.py -v -p -h
python arguments.py -h -phv

notice that the order of the arguments are irrelevant

7. Tutorial

copy the following program and call it client.py

#!/usr/bin/python

import sys
from socket import *
serverHost = ’localhost’
serverPort = 2000

# create a TCP socket
s = socket(AF_INET, SOCK_STREAM)


s.connect((serverHost, serverPort))
s.send(’Hello world’)
data = s.recv(1024)
print data

now copy this program and call it server.py

#!/usr/bin/python

from socket import *
myHost = ’’
myPort = 2000

# create a TCP socket
s = socket(AF_INET, SOCK_STREAM)
# bind it to the server port number
s.bind((myHost, myPort))
# allow 5 pending connections
s.listen(5)


while 1:
    # wait for next client to connect
    connection, address = s.accept()
    while 1:
        data = connection.recv(1024)
        if data:
            connection.send(’echo -> ’ + data)
        else:
            break
    connection.close()

now run server.py by opening up a terminal and typing:
```
python server.py
```
now open up another terminal and type:
```
python client.py
```
now modify your client.py file so that you connect to your neighboughs machine (hint you can find the name of your neighbough machine by opening up a terminal and typing):
```
hostname
```
and adding 1 or subtracting 1 from the end value
this machine name needs to replace the string localhost in client.py
now rerun your client.py program
now modify your server program so that it uses the string to attempt to read a file and return its contents (choose the filename README )
- using an editor create the file README (notice python is case sensitive on GNU/Linux so use capitals for this example)
- hint see earlier section on file handling
now stop your local server from running and restart it
modify your client program so that it connects to localhost and not your neighboughs machine
- and debug your code
when your code works modify your client to connect to your neighboughs machine

Index

1. Python Gotya’s
2. Python and file handling
3. Further Python Networking
4. IMAP library
5. moppsy
6. Tutorial
7. Tutorial
Index

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