Exception Handling and Files

1. Exception Handling and Files CSIS 1595: Fundamentals of Programming and Problem Solving 1

2. File Access Problems • Files may not be usable by program for many reasons • File program reads from may not exist • Data in files may be wrong format • File program writes to may not be accessible • Operating system may just not feel like it… • All of these cause exception if not handled

3. Exception Handling Syntax try: code that might cause exception other code that should not be run if exception occurs except exception type: code to run if exception occurs …

4. Key Python Exceptions • Key idea: Exception handling only way to detect and handle some types of errors • Parsing non-numeric values • Accessing nonexistent files

5. Simple Example

6. Exception Handling and Design • Possible actions if file can’t be opened: • Shut down program (if file necessary) • Prompt for another file (if user entered filename) • Must also give user option to quit

7. Exception Handling and Design • Possible actions if file can’t be opened: • Use default values (often “empty document”)

8. Exception Handling During Run • Failure can happen at any point in program • File on external drive opened successfully • User unplugs drive in middle of program… • Should enclose all file access in try block

9. Nested Exceptions • Program reads numbers from file, prints total • Reading from file  IOError • Parsing numbers to add to total  ValueError • Key question: How to handle each? • Can’t open file  Quit program • Number can’t be parsed  Skip it and keep running (and print warning) • Often done by nesting try blocks • Outer block handles file problems, exiting program • Inner block handles parsing problems, jumping to next iteration of loop

10. Nested Exceptions

11. Handling Exceptions in Caller • What if exception nothandled inside function? • Immediately exit function • Exception then sent to caller who can then handle it • Simple way to force program exit for exceptions in functions