The document discusses exception handling in Python programming. It defines an exception as an event that occurs during program execution that indicates an error. It describes how Python uses try and except blocks to handle exceptions. The try block contains code that may raise exceptions, and except blocks handle specific exceptions. Finally blocks always execute to cleanup resources, even if no exception occurs. User-defined exceptions should inherit from the built-in Exception class. The raise statement throws exceptions, and assertions act like raise-if statements to validate program logic.

1. PYTHON PROGRAMMING
Engr. Ranel O. Padon
X. EXCEPTION HANDLING

2. PYTHON PROGRAMMING TOPICS
I
• Introduction to Python Programming
II
• Python Basics
III
• Controlling the Program Flow
IV
• Program Components: Functions, Classes, Packages, and Modules
V
• Sequences (List and Tuples), and Dictionaries
VI
• Object-Based Programming: Classes and Objects
VII
• Customizing Classes and Operator Overloading
VIII
• Object-Oriented Programming: Inheritance and Polymorphism
IX
• Randomization Algorithms
X
• Exception Handling and Assertions
XI
• String Manipulation and Regular Expressions
XII
• File Handling and Processing
XIII
• GUI Programming Using Tkinter

5. try
except
Defensive
Programming
else
finally
raise
assert
class Exception

7. http://www.newser.com/story/20282/china-atm-glitch-nets-man-life.html

8. http://www.newser.com/story/148427/atm-error-lets-man-withdraw-15m-gamble-it-away.html

9. INTRODUCTION
Exception is an indication of a special event that
occurs during a program’s execution.
Exception indicates that, although the event can occur,
the event occurs infrequently.

10. INTRODUCTION

11. INTRODUCTION
When a Python script encounters a situation that
it can't cope with, it raises an exception.
An exception is a Python object that represents an error.

12. INTRODUCTION

13. INTRODUCTION

14. INTRODUCTION | Exceptions
1.
2.
3.
4.
5.
6.
7.
8.
9.
Division by Zero
Addition of two incompatible types.
Accessing a file that is nonexistent.
Accessing a nonexistent index of a sequence.
Deleting a table in a disconnected database server.
Withdrawing money greater than the available amount.
Invalid input bearing angle.
Negative perimeter distance.
…

15. INTRODUCTION | Exceptions
Resource Leak
Aborting a program component could leave a file or a network
connection in a state in which other programs
are not able to acquire the resource.

16. EXCEPTION HANDLING

17. EXCEPTION HANDLING
Advantages
1. handle the error
- catching the exception
- resolving the exception
2. continue processing as if no error had occured
3. programs are more clear, robust and more fault-tolerant

18. EXCEPTION HANDLING
When no exceptions occur, exception-handling code
incurs little or no performance penalties.
Programs that implement exception handling operate more
efficiently than programs that perform error handling throughout
the program logic.

19. EXCEPTION HANDLING
Brute-Force Error Handling

20. EXCEPTION HANDLING
Brute-Force Error Handling
it may work, but intermixing the logic of the program
with the error-handling logic can make the program difficult
to read, modify, maintain and debug—especially in large
applications.

21. EXCEPTION HANDLING
Exception handling enables the programmer to
remove error-handling code from the “main line”
of the program’s execution.
This improves program clarity and enhances modifiability.

22. EXCEPTION HANDLING | Python
Mechanism is similar with that used in Modula-3, C# and Java.
Not all programming languages support exception handling.

23. EXCEPTION HANDLING

24. EXCEPTION HANDLING | try-except

25. EXCEPTION HANDLING | try-except
Python uses try statements to enable exception handling.
The try statement encloses statements
that potentially cause exceptions.
A try statement consists of keyword try, followed by a colon
(:), followed by a suite of code in which exceptions may occur,
followed by one or more clauses.

26. EXCEPTION HANDLING | try-except
Immediately following the try suite may be
one or more except clauses
(also called except handlers).
Each except clause specifies zero or more
exception names that represent the type(s)
of exceptions the except clause can handle.

27. EXCEPTION HANDLING | try-except
If an exception occurs in a try suite,
the try suite expires and program control
transfers to the first matching except handler
(if there is one) following the try suite.

28. EXCEPTION HANDLING | try-except
an except handler always should specify the name
of the exception to catch.
an empty except handler should be used only for a
default catch-all case.

29. EXCEPTION HANDLING | try-except
If no exceptions occur in a try suite,
the interpreter ignores the exception handlers
for that try statement.

30. EXCEPTION HANDLING | try-except

31. EXCEPTION HANDLING | else
After the last except clause, an optional else clause
contains code that executes if the code in
the try suite raised no exceptions.

32. EXCEPTION HANDLING | Summary

33. EXCEPTION HANDLING | Summary

34. EXCEPTION HANDLING | Summary

35. EXCEPTION HANDLING | Summary

36. EXCEPTION HANDLING | finally
A try suite can be followed by zero except clauses;
in that case, it must be followed by a finally clause.
The code in the finally suite always executes,
regardless of whether an exception occurs.

37. EXCEPTION HANDLING | finally
Programs frequently request and release resources dynamically.
Programs that obtain certain types of resources (such as files)
sometimes must return those resources explicitly to the system
to avoid resource leaks.

38. EXCEPTION HANDLING | finally
The finally clause is an ideal location to place
resource deallocation code for resources acquired.

39. EXCEPTION HANDLING | finally

40. EXCEPTION HANDLING | finally

41. EXCEPTION HANDLING | finally

42. EXCEPTION HANDLING | finally

43. EXCEPTION HANDLING | Exception
Exceptions are objects of classes that inherit
from class Exception.
Programmer-defined exception classes should
derive directly or indirectly from class Exception.

44. class Exception

45. class Exception
(1/2)

46. class Exception
(2/2)

47. EXCEPTION HANDLING | raise
Raising/Throwing An Exception
raise keyword
Catching and handling exceptions enables a
program to know when an error has occurred, then to take
actions to minimize the consequences of that error.

48. EXCEPTION HANDLING | raise
The raise statement may specify an argument or
arguments that initialize the exception object.
In this case, a comma follows the exception name, and the
argument or a tuple of arguments follows the comma.

49. EXCEPTION HANDLING | raise Args

50. EXCEPTION HANDLING | raise Args

51. EXCEPTION HANDLING | Custom

52. EXCEPTION HANDLING | Custom

53. EXCEPTION HANDLING | Custom

54. EXCEPTION HANDLING | assert
An assertion is a sanity-check that you can turn on/off
when you are done with your testing of the program.
Assertion is like a raise-if statement
(or to be more accurate, a raise-if-not statement).
An expression is tested, and if the result comes up false,
an exception is raised.

55. EXCEPTION HANDLING | assert
Programmers often place assertions
at the start of a function to check for valid input,
and after a function call to check for valid output.

56. EXCEPTION HANDLING | assert

57. EXCEPTION HANDLING | assert

58. EXCEPTION HANDLING | assert

59. EXCEPTION HANDLING | assert

60. EXCEPTION HANDLING | assert

61. EXCEPTION HANDLING | assert

62. EXCEPTION HANDLING

63. EXCEPTION HANDLING

64. REFERENCES
 Deitel, Deitel, Liperi, and Wiedermann - Python: How to Program (2001).
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knowledge. 

65. Thank You!