Introduction to Python

1
Introduction to Python

• Recursion
• Exceptions

2
Recursive Function

• Definition
• A function which calls itself
• Basic Example
• Calculate factorial
• Example
• def factorial(n)
• if n 1
• return 1
• else
• return nfactorial(n-1)

3
Recursive Example

• Factorial

factorial(5)
120
24
6
2
1
4
Recursion

• Key parts
• Termination Condition
• Call to itself

def factorial(n) if n 1 return
1 else return nfactorial(n-1)
Termination Condition
Call to itself
5
Uses

• Traversing Trees

def inorder(node) if node None
return inorder(node.left) print
node.value, inorder(node.right) 1 3 6 7 9 10 12
15
6
Uses

• Traversing Trees

def preorder(node) if node None
return print node.value, inorder(node.left)
inorder(node.right) 10 3 1 7 6 9 12 15
7
Uses

• Sorting
• Quick Sort
• Bubble Sort

def quicksort(values, low, high) if (high
low) pivot partition(values, low,
high) quicksort(values, low, pivot-1) quicksor
t(values, pivot1, high)
8
Uses

• Puzzles
• Tower of Hanoi
• Fractals
• Koch Snowflake

9
Exceptions

• Definition
• Errors detected during execution
• Basic Example
• Divide by zero
• 1 / 0
• Traceback (most recent call last)
• File "", line 1, in ?
• 1 / 0
• ZeroDivisionError integer division or modulo by
  zero

10
Exceptions

• Motivation
• Move error handling code away from main code
  block
• Deal with exceptional cases separately
• How it works
• Exceptions are thrown (or raised) and caught
• Control exits the current code block when the
  exception is thrown
• An exception can then be caught by a catching
  code block

11
Exceptions

• Throwing
• Many common operations may throw an exception
• List index out of bounds
• Invalid type conversions
• Exceptions can be thrown manually using the raise
  keyword
• raise ValueError, "Bad Value
• Catching
• Thrown exceptions must be caught
• If the exception is never caught the progam will
  terminate

12
Exceptions

• Handling Syntax
• try
• except
• except
• except
• else

13
Exceptions

• Example
• try
• x 1 / 0
• except ZeroDivisionError
• print 'Divide by zero error'
• Divide by zero error

14
Exceptions

• Example
• try
• x 1 / 0
• except IOError
• print 'Input/Output error'
• except
• print 'Unknown error'
• Unknown error

15
Exceptions

• Types of Exceptions
• There is a hierarchy of exceptions
• All built-in exceptions are derived from
  Exception
• An exception will be caught by any type higher up
  in the hierarchy

Exception
StandardError
ArithmeticError
ZeroDivisionError
SystemExit
ValueError
OverflowError
StopIteration
LookupError
IndexError
KeyError
16
Exceptions

• Example
• try
• x 1 / 0
• except Exception
• print 'Exception caught'
• Exception caught

17
Exceptions

• Propagation
• If no proper except block can be found in the
  current block, the exception propagates back to
  the calling function
• def func1()
• try
• a 1 / 0
• except ValueError
• print 'first'
• def func2()
• try
• func1()
• except
• print 'second'
• func2()
• second

18
Exceptions

• Example
• try
• x 1 / 0
• except Exception
• print 'Exception caught'
• Exception caught

19
Command Line Arguments

• Variables can be passed to the program via
  command line arguments
• Values are stored in sys.argv
• First value is sys.argv is always the program
  name
• Preferable to prompting user for input after
  program has begun
• Example

Command Line
test.py

• python test.py
• 'test.py'
• python test.py 2 3 words
• 'test.py', '2', '3', 'words'