Presentation is loading. Please wait.

Presentation is loading. Please wait.

Lecture 07 – Exceptions.  Understand the flow of control that occurs with exceptions  try, except, finally  Use exceptions to handle unexpected runtime.

Published byMilton Boyd Modified over 9 years ago

Similar presentations

Presentation on theme: "Lecture 07 – Exceptions.  Understand the flow of control that occurs with exceptions  try, except, finally  Use exceptions to handle unexpected runtime."— Presentation transcript:

1 Lecture 07 – Exceptions

2  Understand the flow of control that occurs with exceptions  try, except, finally  Use exceptions to handle unexpected runtime errors gracefully  'catching' an exception of the appropriate type  Generate exceptions when appropriate  raise an exception 2COMPSCI 107 - Computer Science Fundamentals

3  Typical code with no errors 3COMPSCI 107 - Computer Science Fundamentals def divide(a, b): result = a / b return result x = divide(5, 5) print(x)

4  What if the function is passed a value that causes a divide by zero?  Error caused at runtime  Error occurs within the function  Problem is with the input  What can we do? 4COMPSCI 107 - Computer Science Fundamentals def divide(a, b): result = a / b return result x = divide(5, 0) print(x)

5  Check for valid input first  Only accept input where the divisor is non-zero 5COMPSCI 107 - Computer Science Fundamentals def divide(a, b): if b == 0: result = 'Error: cannot divide by zero' else: result = a / b return result x = divide(5, 0) print(x)

6  Check for valid input first  What if b is not a number? 6COMPSCI 107 - Computer Science Fundamentals def divide(a, b): if (type(b) is not int and type(b) is not float): result = "Error: divisor is not a number" elif b == 0: result = 'Error: cannot divide by zero' else: result = a / b return result x = divide(5, 'hello') print(x)

7  Check for valid input first  What if a is not a number? 7COMPSCI 107 - Computer Science Fundamentals def divide(a, b): if (type(b) is not int and type(b) is not float or type(a) is not int and type (a) is not float): result = ('Error: one or more operands' + ' is not a number') elif b != 0: result = a / b else: result = 'Error: cannot divide by zero' return result x = divide(5, 'hello') print(x)

8  Code that might create a runtime error is enclosed in a try block  Statements are executed sequentially as normal  If an error occurs then the remainder of the code is skipped  The code starts executing again at the except clause  The exception is "caught" 8COMPSCI 107 - Computer Science Fundamentals try: statement block except: exception handling statements

9  Assume input is correct  Deal with invalid input an an exceptional case 9COMPSCI 107 - Computer Science Fundamentals def divide(a, b): try: result = a / b except: result = 'Error in input data' return result x = divide(5, 'hello') print(x)

10  What is the output of the following? 10COMPSCI 107 - Computer Science Fundamentals def divide(dividend, divisor): try: quotient = dividend / divsor except: quotient = 'Error in input data' return quotient x = divide(5, 0) print(x) x = divide('hello', 'world') print(x) x = divide(5, 5) print(x)

11  The general except clause catching all runtime errors  Sometimes that can hide problems 11COMPSCI 107 - Computer Science Fundamentals def divide(dividend, divisor): try: quotient = dividend / divsor except: quotient = 'Error in input data' return quotient x = divide(5, 2.5) print(x)

12  Can choose to catch only some exceptions 12COMPSCI 107 - Computer Science Fundamentals def divide(a, b): try: result = a / b except TypeError: result = 'Type of operands is incorrect' except ZeroDivisionError: result = 'Divided by zero' return result x = divide('hello', 0) print(x)

13  Any kind of built-in error can be caught  Check the Python documentation for the complete list 13COMPSCI 107 - Computer Science Fundamentals BaseException +-- SystemExit +-- KeyboardInterrupt +-- GeneratorExit +-- Exception +-- StopIteration +-- ArithmeticError | +-- FloatingPointError | +-- OverflowError | +-- ZeroDivisionError +-- AssertionError +-- AttributeError +-- BufferError +-- EOFError +-- ImportError +-- LookupError | +-- IndexError | +-- KeyError +-- MemoryError +-- NameError | +-- UnboundLocalError +-- OSError | +-- BlockingIOError | +-- ChildProcessError | +-- ConnectionError | | +-- BrokenPipeError | | +-- ConnectionAbortedError | | +-- ConnectionRefusedError | | +-- ConnectionResetError | +-- FileExistsError | +-- FileNotFoundError | +-- InterruptedError | +-- IsADirectoryError | +-- NotADirectoryError | +-- PermissionError | +-- ProcessLookupError | +-- TimeoutError +-- ReferenceError +-- RuntimeError | +-- NotImplementedError +-- SyntaxError | +-- IndentationError | +-- TabError +-- SystemError +-- TypeError +-- ValueError | +-- UnicodeError | +-- UnicodeDecodeError | +-- UnicodeEncodeError | +-- UnicodeTranslateError

14  raise  Creates a runtime error – by default, the most recent runtime error  raise Error('Error message goes here') 14COMPSCI 107 - Computer Science Fundamentals try: statement block here except: more statements here (undo operations) raise raise ValueError("My very own runtime error")

15  Given the following function that converts a day number into the name of a day of the week, write code that will generate an informative exception if the name is outside the desired range. 15COMPSCI 107 - Computer Science Fundamentals def weekday(n): names = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'] return names[n]

16  Given the following function that converts a day number into the name of a day of the week, write code that will generate an informative exception if the name is outside the desired range. 16COMPSCI 107 - Computer Science Fundamentals def weekday(n): if n not in range(0, 8): raise ValueError('Data must be between 0 and 7 inclusive') names = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'] return names[n]

17  Modify the following function that calculates the mean value of a list of numbers to ensure that the function generates an informative exception when input is unexpected 17COMPSCI 107 - Computer Science Fundamentals def mean(data): sum = 0 for element in data: sum += element mean = sum / len(data) return mean

18  finally  Executed after the try and except blocks, but before the entire try-except ends  Often used with files to close the file  else  Executed only if the try clause completes with no errors 18COMPSCI 107 - Computer Science Fundamentals try: statement block here except: more statements here (undo operations) finally: more statements here (close operations) try: statement block here except: more statements here (undo operations) else: more statements here (close operations)

19 19COMPSCI 107 - Computer Science Fundamentals try: file = open('test.txt') file.write('Trying to write to a read-only file') except IOError: print('Error: cant find file or read data') else: print("Written content in the file successfully") finally: file.close()

20  What is the output of the following program when x is 1, 0 and '0'? 20COMPSCI 107 - Computer Science Fundamentals try: print('Trying some code') 2 / x except ZeroDivisionError: print('ZeroDivisionError raised here') except: print('Error raised here') raise else: print('Else clause') finally: print('Finally')

21  Exceptions alter the flow of control  When an exception is raised, execution stops  When the exception is caught, execution starts again  try… except blocks are used to handle problem code  Can ensure that code fails gracefully  Can ensure input is acceptable  raise  Creates a runtime exception  finally  Executes code after the exception handling code 21COMPSCI 107 - Computer Science Fundamentals

Download ppt "Lecture 07 – Exceptions.  Understand the flow of control that occurs with exceptions  try, except, finally  Use exceptions to handle unexpected runtime."

Similar presentations

1992-2007 Pearson Education, Inc. All rights reserved. 1.. Exception Handling.

Pearson Education, Inc. All rights reserved. 1.. Exception Handling.
1 Exceptions: An OO Way for Handling Errors Rajkumar Buyya Grid Computing and Distributed Systems (GRIDS) Laboratory Dept. of Computer Science and Software.

1 Exceptions: An OO Way for Handling Errors Rajkumar Buyya Grid Computing and Distributed Systems (GRIDS) Laboratory Dept. of Computer Science and Software.
Exception Handling Genome 559. Review - classes 1) Class constructors - class myClass: def __init__(self, arg1, arg2): self.var1 = arg1 self.var2 = arg2.

Exception Handling Genome 559. Review - classes 1) Class constructors - class myClass: def __init__(self, arg1, arg2): self.var1 = arg1 self.var2 = arg2.
Exception Handling The purpose of exception handling is to permit the program to catch and handle errors rather than letting the error occur and suffer.

Exception Handling The purpose of exception handling is to permit the program to catch and handle errors rather than letting the error occur and suffer.
C++ Programming: Program Design Including Data Structures, Fourth Edition Chapter 15: Exception Handling.

C++ Programming: Program Design Including Data Structures, Fourth Edition Chapter 15: Exception Handling.
Chapter 16: Exception Handling C++ Programming: From Problem Analysis to Program Design, Fifth Edition.

Chapter 16: Exception Handling C++ Programming: From Problem Analysis to Program Design, Fifth Edition.
Objectives In this chapter you will: Learn what an exception is Learn how to handle exceptions within a program See how a try / catch block is used to.

Objectives In this chapter you will: Learn what an exception is Learn how to handle exceptions within a program See how a try / catch block is used to.
C++ Programming: From Problem Analysis to Program Design, Third Edition Chapter 16: Exception Handling.

C++ Programming: From Problem Analysis to Program Design, Third Edition Chapter 16: Exception Handling.
Slides prepared by Rose Williams, Binghamton University ICS201 Exception Handling University of Hail College of Computer Science and Engineering Department.

Slides prepared by Rose Williams, Binghamton University ICS201 Exception Handling University of Hail College of Computer Science and Engineering Department.
Introduction to Computing Using Python Exceptions (Oops! When things go wrong)  Errors and Exceptions  Syntax errors  State (execution) errors.

Introduction to Computing Using Python Exceptions (Oops! When things go wrong)  Errors and Exceptions  Syntax errors  State (execution) errors.
Lecture 07 – Exceptions.  Understand the flow of control that occurs with exceptions  try, except, finally  Use exceptions to handle unexpected runtime.

Lecture 07 – Exceptions.  Understand the flow of control that occurs with exceptions  try, except, finally  Use exceptions to handle unexpected runtime.
An Introduction to Python and Its Use in Bioinformatics

An Introduction to Python and Its Use in Bioinformatics
EXCEPTIONS Def: An exception is a run-time error. Examples include: attempting to divide by zero, or manipulate invalid data.

EXCEPTIONS Def: An exception is a run-time error. Examples include: attempting to divide by zero, or manipulate invalid data.
 2002 Prentice Hall. All rights reserved. 1 12.3 Exception-Handling Overview Exception handling –improves program clarity and modifiability by removing.

 2002 Prentice Hall. All rights reserved Exception-Handling Overview Exception handling –improves program clarity and modifiability by removing.
Exception Handling Recitation – 10/(23,24)/2008 CS 180 Department of Computer Science, Purdue University.

Exception Handling Recitation – 10/(23,24)/2008 CS 180 Department of Computer Science, Purdue University.
Exceptions COMPSCI 105 S2 2015 Principles of Computer Science.

Exceptions COMPSCI 105 S Principles of Computer Science.
Python Crash Course Programming Bachelors V1.0 dd 13-01-2015 Hour 5.

Python Crash Course Programming Bachelors V1.0 dd Hour 5.
Python Mini-Course University of Oklahoma Department of Psychology Lesson 19 Handling Exceptions 6/09/09 Python Mini-Course: Lesson 19 1.

Python Mini-Course University of Oklahoma Department of Psychology Lesson 19 Handling Exceptions 6/09/09 Python Mini-Course: Lesson 19 1.
UNIT 3 TEMPLATE AND EXCEPTION HANDLING. Introduction  Program errors are also referred to as program bugs.  A C program may have one or more of four.

UNIT 3 TEMPLATE AND EXCEPTION HANDLING. Introduction  Program errors are also referred to as program bugs.  A C program may have one or more of four.
WEEK 14 - 15 EXCEPTION HANDLING. Syntax Errors Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while.

WEEK EXCEPTION HANDLING. Syntax Errors Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while.

Similar presentations

Ads by Google