1. Generic file reading function

often we want to read a file and process it a line at a time
- we could write a function easily enough..
- but we do this many times
another way is to generate a library function which reads in the file, and calls another function a line at a time

#!/usr/bin/python

from sys import argv

def scanner(name, function):
    file = open(name, ’r’)
    line = file.readline()
    while line:
        function(line)
        line = file.readline()
    file.close()

def processLine(line):
    print line


filename = ’ftplog.data’
scanner(filename, processLine)

2. imap library and Python

the Internet Message Access Protocol (commonly known as IMAP or IMAP4) is an application layer Internet protocol that allows a local client to access email on a remote server
in Python we can use the imaplib to download email

#!/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()

in this program the m.search function returns two values: typ and data
typ is a status code and data is a list of strings, the first string is a space separated list of numbers indicating the available messages
hence the construct for num in string.split(data[0]): extracts each number in sequence and assigns it to num

3. Spam and email

your task is to write a spam filter in Python
this will use networking principles as discussed in lectures
and use Python modules to draw on the services which sit above TCP
your program needs to download email messages from moppsy.comp.glam.ac.uk
- perform a spam test on the message
- if it is not spam then it needs to send it to a local smtp port
finally you need to document your program and suggest improvements

4. 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" u012345678@localhost
this is the body of the message
.
$ exit

5. Skeleton solution

you can use this as a basis for your submission skeleton

6. Strings

the string module is very important
- many tasks involve string handling
network protocols also requires good string manipulation
- http is string based: GET, PUT etc
- smtp is also string based: Subject, From, To, fields etc
string module provides functions to
- split strings into words
- strip newlines off the end of a string
- find substrings
consider reading through the online Python documentation and examine the string module

7. Coursework skeleton snippet

#
#  isSpam - returns 1 if the message looks like spam
#           returns 0 if the message looks ok
#


def isSpam(message):
    total = 0
    htmlFound = 0
    for line in StringIO(getBody(message)).readlines():
        print line
    # incomplete

#
# postEmail - sends message to the smtp port
#


def postEmail(message):
    print "message from", getField(message, "From:")
    print "subject is", getField(message, "Subject:")
    # incomplete

#
#  handleEmail - tests whether the message is spam or not.
#                If spam then it adds it to a file
#                otherwise post it to the smtp port
#


def handleEmail(message):
    if (isSpam(message)):
        print "message is spam"
    else:
        print "message is good"
        postEmail(message)
    # incomplete

#
#  collectEmail - retrieves the email from the imap server
#

def collectEmail():
    m = imaplib.IMAP4_SSL(’moppsy.comp.glam.ac.uk’)
    m.login(getpass.getuser(), ’yourpasswordgoeshere’)
    m.select()
    typ, data = m.search(None, ’ALL’)
    print data
    for num in string.split(data[0]):
        print num
        typ, data = m.fetch(num, ’(RFC822)’)
        handleEmail(data[0][1])
    m.logout()

collectEmail()

8. Tutorial Assignment support

divide and conqueror is the classic Computer Science method for solving large projects
- in our case the spam filter coursework could be split up into three areas
  - downloading the emails from the remote server
  - detecting for the presence of spam (>=60% of lines contain an HTML tag)
  - posting non spam emails locally (using smtp - see last weeks tutorial)

9. Downloading emails from the remote server

note be careful, the following examples use bold font for your keyboard input and normal courier for terminal output
- obviously you need to subsitute u012345678 for your real enrolment number
open up a terminal and login to the imap server moppsy.comp.glam.ac.uk and send your self a test email message. You can do this by:

$ ssh moppsy.comp.glam.ac.uk
login: u012345678
Password:
$ mail -s "this is a test" u012345678@localhost
this is the body of the message
.
CC: press the enter key here
$ exit

now test whether your email message is resident on the server by:

ssh moppsy.comp.glam.ac.uk
login: u012345678
Password:
$ mail
Mail version 8.1.2 01/15/2001.  Type ? for help.
"/var/mail/u012345678": 1 message 1 new
N 1 u012345678@localhost  Wed Nov 15 13:30  this is a test
& x
$ exit

now cut and paste this program into a file called testimap.py

#!/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()

check it works..
now download the skeleton program and run it
check that it prints your test message

Index

1. Generic file reading function
2. imap library and Python
3. Spam and email
4. moppsy
5. Skeleton solution
6. Strings
7. Coursework skeleton snippet
8. Tutorial Assignment support
9. Downloading emails from the remote server
Index

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