Show understanding of an exception and the importance of exception handling

20.2 File Processing and Exception Handling

What is an Exception?

An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions. It is typically caused by an error condition that the program cannot handle automatically, such as trying to read a file that does not exist or dividing by zero.

Why is Exception Handling Important?

• Prevents program crashes by providing a controlled response to error conditions.
• Allows resources (e.g., file handles, network connections) to be released safely.
• Improves user experience by presenting clear error messages instead of cryptic system failures.
• Facilitates debugging by isolating the location and type of the error.

Typical Exceptions in File Processing

Structure of Exception Handling (Python‑style pseudocode)

1. Identify the block of code that may raise an exception.
2. Wrap that block in a try statement.
3. Provide one or more except clauses to catch specific exceptions.
4. Optionally include a else clause that runs if no exception occurs.
5. Use a finally clause to release resources regardless of success or failure.

Example: Reading a Text File Safely

try:
file = open('data.txt', 'r')
for line in file:
process(line)
except FileNotFoundError:
print('Error: data.txt was not found.')
except IOError as e:
print('I/O error:', e)
else:
print('File processed successfully.')
finally:
if 'file' in locals():
file.close()

Flow Comparison: Normal Execution vs. Exception Flow

Custom Exceptions

When the built‑in exception types are not descriptive enough, you can define your own exception class.

class InvalidRecordError(Exception):
"""Raised when a record in a data file does not meet validation rules."""
pass
# Usage
if not validate(record):
raise InvalidRecordError('Record format is incorrect')

Best Practices for Exception Handling in File Processing

• Catch the most specific exception possible; avoid a bare except: unless re‑raising.
• Never suppress an exception silently; at least log the error.
• Always close files (or other resources) in a finally block or use a context manager.
• Validate input data before attempting file operations to reduce the chance of exceptions.
• Provide user‑friendly messages that suggest corrective action.

Key Takeaway

Exception handling transforms unpredictable runtime errors into manageable events, allowing programs that work with files to remain robust, maintainable, and user‑oriented.