This document provides an introduction to learning Python. It discusses prerequisites for Python, basic Python concepts like variables, data types, operators, conditionals and loops. It also covers functions, files, classes and exceptions handling in Python. The document demonstrates these concepts through examples and exercises learners to practice char frequency counting and Caesar cipher encoding/decoding in Python. It encourages learners to practice more to master the language and provides additional learning resources.

1. Learn Python The Hard Way
Utkarsh Sengar
Contents taken from © Copyright 2010, Zed A. Shaw.

2. Resources
• Learn Python The Hard Way, 2nd Edition
– http://bit.ly/python-lug (Free online)
– Paperback book costs $15.99
– http://learncodethehardway.org
• Google python class:
– http://code.google.com/edu/languages/google-
python-class/
• MIT Open courseware:
– http://academicearth.org/courses/introduction-to-
computer-science-and-programming

3. It’s Easy.
 5 most important things:
– Do Not Copy-Paste
– Code
– Practice
– Practice
– Practice

4. Prerequisites
 Python v2.5 +
 python
 Any text editor.
 gEdit (Linux)
 TextMate (OSX)
 Settings
 Tabs Width: 4
 Insert spaces instead of tabs
 Your Mind.

5. TODO: today
 Total of 52 chapters
 We will cover: 1-22, 28-35, 39-42
 Basic constructs, data structures, OOP
 Solve two simple python problems.
 Homework: Another problem. What did you think?

6. Lets code…..
etherPad
http://ip-address:9001

7. But first……..Pip, pep8, iPython
• pip install simplejson
– For python packages in Python package index
• pep8 script.py
– Python style guide. Your best friend
• Python Easter egg
– import this
• Ipython
• Open pydocs :
docs.python.org/library/index.html

8. comments
 Single line comments only
 '#' (an octothorpe) notifies beginning
 Ends with a newline

9. Printing statements
 print is builtin function
 Usage: print ”Hello, world!”
 Appends a newline automatically
 To avoid new line, use a comma (,)
 Usage: print ”Hello”,
print ”roopesh”

10. Basic math
 * / % + - < > <= >=
 Same as C operator precedence
 / does a integer division
 Example:
 print "Hens", 25 + 30 / 6
 print 3 + 2 + 1 - 5 + 4 % 2 - 1 / 4 + 6
 print "What is 5 - 7?", 5 – 7
 print "Is it greater or equal?", 5 >= -2

11. More operators
• Basic ones: + - * / %
• Try:
– x // y
– abs(x)
– int(x), long(x), float(x)
– complex(re, im)
– c.conjugate()
– pow(x, y) or x ** y

12. variables
 No need of types
 Duck typing
 cars = 100
 space_in_car = 4.0
 result = cars + space_in_car
 cars_not_driven = cars
 Can assign one var to another
 Tip: type(cars)

13. Variables and printing
 Can use placeholders in print statements
 print ”let's talk about %s” % my_name
 print "There are %d types of people." % 10
 print "Those who know %s and those who %s." %
(binary, do_not)
 Also can use names for placeholders
 print ”my name is %(name)s” % {'name':
my_name}

14. Text (String)
 There are 3 types of notation
 Single quotes, double quotes, multiline text
 Multiline text stores the text, whitespaces and escape
characters as well
 val = ’hello world’
 val = ”hello again!”
 val = ”Hello n
World
!!”
 val = ”””no need to ’escape’ anything – ”ok?” ”””
 Interesting functions:
upper(), lower(), title(), swapcase(),strip()

15. Booleans
 True and False
 Can use in print with placeholders as well
 Anything that has a value is true, anything like
0, zero length string, zero length list or dictionary
is false
 val = ’Hi my friend’
if val:
print ”yo!”
 If ’Hi’ in val:
print ’this keeps getting better!’

16. Input
 There are two functions: input(), raw_input()
 raw_input() contains string and input() could
contain object
 first = raw_input("Please enter your age ")
second = input("Please enter your age again")
 print "You said you are", first
print "Then you said you are", second
 Try again, pass 40+2 in second

17. Working with python prog files
 Any arguments passed to python prog files is
stored in argv
 Zero based arguments, first one is always
script name
 import sys
print sys.argv[0]
print sys.argv[1]
 Run: python file.py 4

18. Working with files
 open(filename, mode) : returns file handle
 read() : to read the whole file
 readline() and readlines(): to read each line
 write(data) : writes to file
 close() : to close the file handle
 from sys import argv
script, filename = argv
txt = open(filename, 'r+') #r or w
print "Here's your script %r:" % script
print "Here's your file %r:" % filename
print txt.read()
txt.write('42')
txt.close()

19. functions
 def keyword to declare a function
 Arguments don't need function type
 Can skip return value
 No return implies it returns a None value
 Always do: x is None and not, x == None
 None is similar to null
 To take unlimited number of arguments: use *arg
as argument, it is packing method for arguments

20. functions cont…
 def print_none():pass
 def print_one(arg1):
print ”got one arg: %s” % arg1
 def print_two(arg1, arg2):
print ”got two args: %s, %s” %(arg1, arg2)
 def print_two_2(*args):
print args
return args

21. Logics
 and, or, not, ==, !=, >, >=, <, <=
 and returns last value which makes the
statement true
 or returns first value which makes the
statement false
 Both are short-circuit operator
 test = True
result = test and 'Test is True' or 'Test is False'

22. Data Structures
 List: [1, 2, 3, 2]
 Tuples: (1, 2, 3, 2)
 Set: {1,2,3}
 Dictionary: {’name’ : ’utkarsh’, ’age’ : 42}

23. Lists and loops
 Lists: [1, 2, 3]
 append(elem), extend(list), insert(idx, elem), rem
ove(x), pop(i), count(i), sort(), reverse(),len(l)
 Tuples: (1, 2, 3)
 Difference:
 lists are mutable,
 tuples are immutable
 Ideally lists should have same data types

24. Lists and loops
 for element in list:
print element
 for i in range(i):
print i
 for i, val in enumerate(list):
print i, val
 sorted_list = sorted(list)
 sort(list)
 List comprehensions for ease of use (later..)

25. Lists and loops
 Lists from strings:
 a = ”A B C D E F”.split()
#['A', 'B', 'C', 'D', 'E', 'F’]
a.pop()
' '.join(a) #joins lists to form a string
 List slicing: list[start:end]
 a[0:len(a)]
 a[:4]
 a[3:]
 a[-1]

26. Dictionaries
 {'name': 'Roopesh’, ’age’ : 42}
 Key-value pairs / lookup table
 Can be accessed with a['name']
 Can add new values with same syntax
a['city'] = 'San Jose'
 Can remove an item with 'del'
del a['city']
 Add a bunch using: dict.update(another_dict)

27. Classes and Objects
 Class keyword, inherits from object
 Constructor: def __init__
 All methods take first arg as the instance of
class
 Denoted with word 'self'
 Any number of args have to follow self
def func(self, val)

28. Classes and Objects
 Instantiating classes with ClassName() syntax
 There is no 'new' keyword
 Some examples in code given.
 To override operators, override functions like
__eq__, __lt__, __gt__, __lte__ etc

29. Class example
class MyClass:
answer = 42
def a_method(self):
print “I am a method”
instance = MyClass ()
instance.a_method()
hasattr(MyClass, “answer”)

30. List comprehensions (bonus)
 [each for each in range(100) if each % 2 == 0]
 [some_func(each) for each in range(100)]
 iter(list) produces a list iterator
 Gives ability to get next item with
iterator.next()
 When iterator gets exhausted, it produces
StopIteration exception

31. Handling Exceptions
Try:
… do some handling
except Error as e:
print e
finally:
… do some final clean up

32. Problem 1
Write a function char_freq() that takes a string
and builds a frequency listing of the characters
contained in it. Represent the frequency listing
as a Python dictionary.
Try it with something like
char_freq("abbabcbdbabdbdbabababcbcbab”)

33. Problem 2
In cryptography, a Caesar cipher is a very simple encryption techniques in which each
letter in the plain text is replaced by a letter some fixed number of positions down the
alphabet. For example, with a shift of 3, A would be replaced by D, B would become
E, and so on. The method is named after Julius Caesar, who used it to communicate
with his generals. ROT-13 ("rotate by 13 places") is a widely used example of a Caesar
cipher where the shift is 13. In Python, the key for ROT-13 may be represented by
means of the following dictionary:
key =
{'a':'n', 'b':'o', 'c':'p', 'd':'q', 'e':'r', 'f':'s', 'g':'t', 'h':'u', 'i':'v', 'j':'w', 'k':'x', 'l':'y', 'm':'z', 'n':'
a', 'o':'b', 'p':'c', 'q':'d', 'r':'e', 's':'f', 't':'g', 'u':'h', 'v':'i', 'w':'j', 'x':'k', 'y':'l', 'z':'m', 'A':'N', '
B':'O', 'C':'P', 'D':'Q', 'E':'R', 'F':'S', 'G':'T', 'H':'U', 'I':'V', 'J':'W', 'K':'X', 'L':'Y', 'M':'Z', 'N':'A'
, 'O':'B', 'P':'C', 'Q':'D', 'R':'E', 'S':'F', 'T':'G', 'U':'H', 'V':'I', 'W':'J', 'X':'K', 'Y':'L', 'Z':'M'}
Your task in this exercise is to implement an encoder/decoder of ROT-13. Once you're
done, you will be able to read the following secret message:
Pnrfne pvcure? V zhpu cersre Pnrfne fnynq!

34. Homework
Integer to Roman
numerals converter

35. Want more?
• Go here:
– 15 Exercises to Know A Programming Language:
Part 1
– http://www.knowing.net/index.php/2006/06/16/
15-exercises-to-know-a-programming-language-
part-1

36. A lot more…
List comprehensions, decorators, lambda
functions, effective unit tests, network and web
programming,
Some popular modules like
urlib, simplejson, ElementTree and lxml for xml
parsing, SQLAlchemy, sciPy, NumPy, mechanize

37. At the end……
“I'll say that learning to create software changes
you and makes you different. Not better or
worse, just different.”
“The world needs more weird people who know
how things work and who love to figure it all out.”
~ Zed Shaw
Source: http://learnpythonthehardway.org/book/advice.html