module 6

• This Lecture covers Files and Exception Handling
• Topics covered in this Lecture

  • Opening, using and closing files
  • File opening modes
  • Writing to files
  • Reading entire files, reading lines and bytes
  • Exception handling
  • Exception handlers, termination and continuation
  • Exception names
  • Try and catch/except
  • Else and finally
• The textbook required for this Lecture is Starting Out with Python, 5th Edition (Global) by Tony Gaddis
• Reading the indicated chapter(s) is required before class
• This week covers Chapter 6 - Files and Exceptions from the textbook
• Files

  So far, programs have been entirely "self-contained" with no storage or data kept between each run. Files on the computer's hard drive allow programs to retain data, receive input or store output.
• Types of files programs can use

  • Settings and configuration files
  • "Office Suite" files
  • Media files
  • Cache and cookie files
  • Assets, save files and replays
• Text files

  Simply contain text, encoded in ASCII or Unicode, and can be opened by programs like Notepad. The extension is not necessarily ".txt".
• Binary files

  Not text-based and are designed to only be opened by a program that can use the data in that format. Includes everything that isn't a text file.
• Working with files in a program

  1. Open the file

  2. Use the file (read, write, or both)

  3. Close the file
• Opening a file

  Requires specifying the filename and the mode to open it in (read, write, append, read/write)
• In Python, the built-in function "open()" opens a file and returns a file object
• Opening a file in different programming languages

  • Python: f = open('file.txt', 'r')

  Java: FileReader f = new FileReader("file.txt");

  PHP: $f = fopen('file.txt', 'r');

  C/C++: FILE *f = fopen("file.txt", "r");

  Ruby: f = File.open('file.txt', 'r')
• Modes in Python

  r (read only), w (write only), a (append), r+ (read and write), w+ (read and write), a+ (append and read/write)

  Adding "b" after a mode opens the file in binary mode
• Illustrating different file modes in Python

  • r: Error when trying to write

  1. File created/emptied, "an" written to start
  2. "an" added to end

  r+: "an" written to start

  w+: "an" written to start

  a+: "an" added to end
• Writing to a file

  Use the "write" method/function of the file object

  Can insert line breaks using "\n"
• Closing a file

  Must be done to ensure data is saved and the file is unlocked for other programs
• The "Test Generator" case study was reviewed from last week
• Modifying the "Test Generator" program to write output to a file

  Prompt user for + or -

  Create and open "output.txt" file in write mode

  Write test type, name, and questions to the file

  Close the file

  Print message informing user of the file
• Expressions

  • 1 + 8
  • 9 + 2
  • 5 + 4
  • 6 + 4
  • 7 + 2
  • 5 + 3
  • 2 + 1
  • 9 + 7
• The result of 1 + 8 is 9
• The result of 9 + 2 is 11
• The result of 5 + 4 is 9
• The result of 6 + 4 is 10
• The result of 7 + 2 is 9
• The result of 5 + 3 is 8
• The result of 2 + 1 is 3
• The result of 9 + 7 is 16
• Raised

  Triggered or thrown (depending on the language)
• Termination

  The program may be unable to continue, hence the response will be to display an error message and end in a clean manner
• Continuation

  The program may be able to continue, e.g. re-prompting for input or performing some other action to work around the exception
• Exception handling

  Another control structure, like selection and iteration (loop) statements, that allows you to control the flow of execution in a program when an exception occurs, transferring control to an exception handler
• So far in the unit, we have not handled the majority of exceptions that could occur in the code we have written
• We've assumed that the user enters a number when prompted to enter a number (which can then be converted to an int/float without an exception occurring)
• This has been to keep our code simple and understandable, avoiding added complexity while we are learning the basics
• We have implemented exception handling on occasion, e.g. re-prompting for input until a number is typed
• It is important that you consider (and handle) the exceptions that could potentially occur in the code that you write
• Basic Exception Handling - Termination

  1. Prompt for a number and try to convert it to an integer

  2. If a ValueError exception is raised, print an error message and use sys.exit() to end the program

  3. If the exception is not raised, continue to print "You typed"
• Basic Exception Handling - Continuation

  1. Prompt for a number and try to convert it to an integer

  2. If a ValueError exception is raised, just put an integer of 7 into the num variable

  3. Continue to print "You typed"
• Basic Exception Handling - Continuation

  1. Place the try/except code inside an endless loop

  2. If a ValueError exception is raised, go to the exception handler and print an error message

  3. If the exception is not raised, reach the break statement and end the loop