Index

1. Python verses similar tools

• is more powerful than Tcl
  • applicable to larger systems development
• has cleaner syntax than Perl
  • easier to maintain
• does not compete head on with Java
  • python is a scripting language
  • Java is a systems language like C++

2. Python can be simple

• #!/usr/bin/python
  
  print "hello world"

3. Python Modules allow for problem decomposition

• similar to Modula-2

• #!/usr/bin/python
  
  title = "hello world"

  • above is called myfile.py

• #!/usr/bin/python
  
  import myfile
  print myfile.title

• when run prints hello world
  

4. Alternative import

• #!/usr/bin/python
  
  from myfile import title
  print title

• note that all python modules need to be saved as name.py
  • so in our example the module myfile was saved into a file called myfile.py
    

5. Python builtin types

• python contains many builtin types
  • use them..
• builtin objects make simple programs easy to understand
  • lists, dictionaries, exist, don’t reinvent the wheel
• built in objects are more efficient than custom data types

6. Builtin objects

7. Expression operators

8. Strings

• concatenation via +
  • repeated via *

• #!/usr/bin/python
  
  print "hi " * 4

• yields

• hi hi hi hi

9. Slicing

• given a string, S = "hello world"
  • can obtain portion of string via: s[2:5]
  • yields: llo
• first character has index 0
  • and also -11
  • last character index is 10 in this example
  • last character index is also -1
• negative values start at right and move to the left
• strings can be sliced using positive and negative values

10. Using dir

• often you may wish to see what methods a module provides
  • run python interactively

• $ python
  Python 1.5.2
  >>> import string
  >>> dir(string)
  [’capitalize’, ’capwords’, ’center’, ’count’, \
   ’digits’, ’expandtabs’, ’find’, ’hexdigits’, \
   ’index’, ’index_error’, ’join’, ’joinfields’, \
   ’letters’, ’ljust’, ’lower’, ’lowercase’, \
   ’lstrip’, ’maketrans’, ’octdigits’, ’replace’, \
   ’rfind’, ’rindex’, ’rjust’, ’rstrip’, ’split’, \
   ’splitfields’, ’strip’, ’swapcase’, \
   ’upper’, ’uppercase’, ’whitespace’, ’zfill’]

• displays methods available
  • in the above example many methods have been removed for brevity
    

11. Methods and documentation

• python online docs
  • under GNU/Linux
• tutorial/laboratory
  • read through the online tutorial under the web address above
  • read about functions and scope rules
    • name resolution, LGB rule
    • local, global, builtin scope

12. Statements

• assignment, calls, print, if/else/elif, for, while, break/continue
• try, except, raise,
• def, return
  • function definitions and returning values

13. Statements

• class
• assert
• exec
• del
• global

14. Example 8 times table

• #!/usr/bin/python
  
  
  for n in range(1, 13):
      print n, "x 8 =", n*8

• $ python eight.py
  1 x 8 = 8
  2 x 8 = 16
  3 x 8 = 24
  4 x 8 = 32
  5 x 8 = 40
  6 x 8 = 48
  7 x 8 = 56
  8 x 8 = 64
  9 x 8 = 72
  10 x 8 = 80
  11 x 8 = 88
  12 x 8 = 96

15. Example of for loop

• #!/usr/bin/python
  
  
  
  for n in range(2, 10):
      print ’n is’, n
  else:
      print ’finished for loop, n is’, n

• $ ./py7.py
  n is 2
  n is 3
  n is 4
  n is 5
  n is 6
  n is 7
  n is 8
  n is 9
  finished for loop, n is 9

16. Example code

• #!/usr/bin/python
  
  
  for n in range(2, 10):
      print ’n is’, n, ’, range(2, n) is’, range(2, n)
      for x in range (2, n):
          print ’x is’, x, ’, n % x == ’, n % x
          if n % x == 0:
              print n, ’equals’, x, ’*’, n/x
              break
      else:
          print n, ’is a prime number’

17. Output of code with debugging

• n is 2 , range(2, n) is []
  2 is a prime number
  n is 3 , range(2, n) is [2]
  x is 2 , n % x ==  1
  3 is a prime number
  n is 4 , range(2, n) is [2, 3]
  x is 2 , n % x ==  0
  4 equals 2 * 2
  n is 5 , range(2, n) is [2, 3, 4]
  x is 2 , n % x ==  1
  x is 3 , n % x ==  2
  x is 4 , n % x ==  1
  5 is a prime number

18. Output of code without debugging

• $ ./py6.py
  2 is a prime number
  3 is a prime number
  4 equals 2 * 2
  5 is a prime number
  6 equals 2 * 3
  7 is a prime number
  8 equals 2 * 4
  9 equals 3 * 3

19. Killer Application for us at Univ of Glam

• we needed a graphical method to alert new GNU/Linux users to the availability of OpenOffice
  • could write a program in Java, C/Motif, C/gnome etc
• prefer a simple scripting solution

• #!/usr/bin/python
  
  import os.path
  from Tkinter import *
  
  def makebutton(message):
          w = Button(text=message, command=’exit’)
          w.pack()
          w.mainloop()
  
  
  if not os.path.isdir(\
     os.path.expandvars(’$HOME/OpenOffice.org1.0.3’)):
          makebutton("You might be interested \
  in OpenOffice (a word clone) which can be \
  invoked from the RedHat menu")

20. Some simple examples

• finding all lines starting with #

• #!/usr/bin/python
  
  
  import sys
  for line in sys.stdin.readlines():
         if line[0] == ’#’:
                 print line

21. Fifth column of /etc/passwd

• fred:llk:1000:1000:Fred,,,:/home/fred:/bin/bash
  bob:wei:1001:1001:Bob,,,:/home/bob:/bin/bash
  alice:kjh:1002:1002:Alice,,,:/home/alice:/bin/bash
  mary:asd:1003:1003:Mary,,,:/home/mary:/bin/bash

• #!/usr/bin/python
  
  import sys, string
  
  
  for line in open(’/etc/passwd’).readlines():
      words = string.split(line, ’:’)
      if len(words) >= 5:
          print words[4]

• Fred
  Bob
  Alice
  Mary

22. Tutorial

• to undertake these tutorials you will need to refer to the python online docs
• write a program using a while loop
  • to write out the nine times table
• write a program using a for loop
  • to write out the seven times table
• write a program using a function and if else statement
  • to write out the 3 times table

23. Tutorial

• read the manual page about /etc/passwd
  • you can do this by typing:

• $ man 5 passwd

  • in the terminal and pressing ’q’ to quit.
• write a program to extract a list of the usernames on the machine
• write a program to list all users on the machine and also display their UIDs
• now write a program to list all users and display a simple message to say whether their password is held in the /etc/shadow file
  

24. Tutorial

• improve the OpenOffice advert such that an additional button appears, called
  • ‘never tell me again’
  • make this take appropriate action!

Index

This document was produced using groff-1.19.