Write code to perform file-processing operations

20.2 File Processing and Exception Handling

What is File Processing?

File processing means reading data from a file, writing data to a file, or updating existing files. In most programming languages (e.g., Python) we treat a file as a stream of bytes or characters.

Common File Modes

Opening a File – Basic Syntax (Python)

# Open a text file for reading
file = open("data.txt", "r")   # returns a file object

# Read the whole content
content = file.read()

# Always close when done
file.close()

Using with (Context Manager)

The with statement automatically closes the file, even if an error occurs.

with open("data.txt", "r") as f:
    lines = f.readlines()   # list of lines
    for line in lines:
        print(line.strip())

Writing to a File

with open("output.txt", "w") as f:
    f.write("Hello, Cambridge A-Level!")
    f.write(f"The value of pi is approximately {3.14159:.5f}")

Exception Handling – Why It Matters

File operations can fail (file not found, permission denied, disk full). Using try…except lets the program respond gracefully instead of crashing.

Try‑Except‑Finally Pattern

try:
    f = open("missing.txt", "r")
    data = f.read()
except FileNotFoundError:
    print("Sorry, the file does not exist.")
except PermissionError:
    print("You do not have permission to read this file.")
else:
    print("File read successfully.")
    print(f"First 100 characters: {data[:100]}")
finally:
    # This runs no matter what
    if 'f' in locals() and not f.closed:
        f.close()
    print("Cleanup complete.")

Specific Exceptions for File I/O

• FileNotFoundError – file does not exist.
• PermissionError – insufficient rights.
• IsADirectoryError – tried to open a directory as a file.
• IOError (or OSError) – generic I/O problem.

Example: Processing a CSV File

Assume students.csv contains: name,age,grade lines.

import csv

try:
    with open("students.csv", "r", newline='') as csvfile:
        reader = csv.DictReader(csvfile)
        total_age = 0
        count = 0
        for row in reader:
            total_age += int(row["age"])
            count += 1
            print(f"{row['name']} is {row['age']} years old, grade {row['grade']}")
        if count > 0:
            avg = total_age / count
            print(f"Average age: ${avg:.2f}$")
except FileNotFoundError:
    print("Error: students.csv not found.")
except ValueError:
    print("Error: non‑numeric age encountered.")
except Exception as e:
    print(f"Unexpected error: {e}")

Best Practices

1. Always use with when possible – guarantees closure.
2. Catch specific exceptions; avoid bare except:.
3. Provide meaningful error messages to the user.
4. Close resources in a finally block if you don’t use with.
5. When dealing with large files, read line‑by‑line or use chunks to avoid high memory usage.
6. For binary files (images, compiled code) use modes like 'rb' and 'wb'.

Quick Checkpoint

• What is the difference between 'w' and 'a' modes?
• Why is finally useful even when an exception is caught?
• Write one line of code that opens log.txt for appending and writes a timestamp.

End of 20.2 File Processing and Exception Handling notes