Lesson 18: Error Handling

Lesson 18: Error Handling#

Reviewer: Mujtaba - I like the contents that will be covered under this section. The only thing that I am not sure is that the error message handling is an advanced and complex concept. It would be helpful to cover this as one of the last chapters in this series. They need to know the concept of class, list, dictionary, and some other stuff before trying to come up with the errors that they might get during working with those concepts.
Owner: Saban, Michael

  • Exception handling

  • Try/except blocks

  • Common errors

Reviewer: Jason for Saban, Michael
Per the Guidelines for Content the following items are missing:
Overview & Introduction
Prerequisites
Summary
Next steps
Everything else looks fine.

Learning Objectives#

  • Define “exception” and describe the difference between syntax and runtime errors.

  • Predict and interpret traceback messages in Python.

  • Use try/except/else/finally to catch and respond to errors.

  • Raise exceptions with raise ValueError() and handle multiple exception types.

  • Debug basic file handling and data conversion errors safely.

Lesson Outline#

Warm-up (5 min)#

x = int(input("Enter a number: "))
print(10 / x)

  • What happens if you type "zero" or "0"? Let’s look at some error outputs

Type of errors (10 min)#

Category

Example

Fix

Syntax Error

if x > 3 print(x)

Read message and fix code before running

Runtime Error

1/0

handle via try/except

Logic Error

wrong output

test inputs & print statements

  • Have students ID error types in 3 snippets

Basic exception handling (15 min)#

try:
    x = int(input("Enter a number: "))
    print(10 / x)
except ValueError:
    print("Please enter numbers only.")
except ZeroDivisionError:
    print("Cannot divide by zero.")
else:
    print("Success!")
finally:
    print("Done.")

  • Order of except blocks matters

  • else runs only if no error and finally always runs

Hands-on demos (10 min)#

File handling error#

try:
    with open("data.txt") as f:
        print(f.read())
except FileNotFoundError as e:
    print("File not found:", e)

Type error in loop#

nums = ["1", "2", "three"]
total = 0

for n in nums:
    try:  # Why not use if n.isdigit() instead? (prevent vs catch)
        total += int(n)
    except ValueError:
        print(f"Skipped bad value: {n}")

print("Total:", total)

Guided practice (20 min)#

  • Lab 1: Write safe_divide(a, b) that returns a/b or None and prints a helpful message. Test with pairs (4,2), (5,0), ("x", 2).

  • Lab 2: Open numbers.txt. Each line has one value. Sum valid ints and skip invalid lines using try/except. Try using raise ValueError if the file is empty.

Reflection & Assessment (10 min)#

  • What is the difference between detecting and handling errors?

  • When is it better to prevent than to catch?

  • Write a try/except that opens a file and prints "no file found" if missing, otherwise prints its first line.

Optional HW#

  • Modify Lab 2 to log errors to error_log.txt instead of printing them.

  • Research assert and write 2 assertions that prevent invalid inputs.

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