Index

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(’moppsy.comp.glam.ac.uk’)
  m.login(getpass.getuser(), getpass.getpass())
  m.select()
  typ, data = m.search(None, ’ALL’)
  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()

5. Arguments in Python

• getopts , provides a useful method for handling arguments
  • in fact many languages have adopted getopts
  • C, C++, bash and python

6. Autoftp arguments in python

• #!/usr/bin/python
  
  import sys, getopt
  
  def Usage ():
      print "autoftp [-v][-p][-h]"
      sys.exit(0)
  
  
  optlist, list = getopt.getopt(sys.argv[1:], ’:vphf:’)
  print "optlist =", optlist
  print "list =", list
  for opt in optlist:
      print opt
      if opt[0] == ’-h’:
          Usage()
      if opt[0] == ’-f’:
          print "file found"
      if opt[0] == ’-v’:
          print "verbose found"
      if opt[0] == ’-p’:
          print "probeonly found"

• notice that the script fails if an unsupported option is issued

• ./autoftp2.py -x
  ...
  getopt.GetoptError: option -x not recognised

7. Better argument handling

• so we need a way to trap these errors
  • python uses an exception handler for this

• #!/usr/bin/python
  
  import sys, getopt
  
  def Usage ():
      print "autoftp [-v][-p][-h]"
      sys.exit(0)
  
  try:
      optlist, list = getopt.getopt(sys.argv[1:],
                                    ’:vphf:’)
  except getopt.GetoptError:
      Usage()
      print "called exception"
      sys.exit(1)
  
  
  for opt in optlist:
      print opt
      if opt[0] == ’-h’:
          Usage()
      if opt[0] == ’-v’:
          print "verbose found"
      if opt[0] == ’-p’:
          print "probeonly found"
      if opt[0] == ’-f’:
          print "file option found"

• when run it yields the following

• ./autoftp3.py -x
  autoftp [-v][-p][-h]

8. When is a module not a module?

• it is often useful to create a module
  • for yourself and others to use in the future
  • to subdivide the large problem set into a number of smaller modules
• sometimes a module might be able to operate as a stand alone program
  • consider autoftp could be organised as a module

• if __name__ == "__main__":
      main()

• which means run the function main if this module is explicitly invoked by the user
  • note that it is not run if this module was imported
    

9. Example times module

• #!/usr/bin/python
  
  import sys
  
  def multiplyby10(value):
      return value+’0’
  
  
  if __name__ == "__main__":
      if len(sys.argv) == 2:
          print ’testing the times module’
          print multiplyby10(sys.argv[1])

10. Example program

• #!/usr/bin/python
  
  import times, sys
  
  
  if len(sys.argv) == 2:
      print ’importing the times module’
      print times.multiplyby10(sys.argv[1])

• note that the module times takes a string and adds a ’0’ to the left hand side
  • effectively multiply by 10
• note it also uses the if __name__ == condition which only calls the multiply routine if this module was invoked as the main program by the user

• ./prog.py 12
  importing the times module
  120

• ./times.py 12
  testing the times module
  120

• exercise for the reader, add a function to perform divide and modulus of a numerical integer string

Index

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