Exception Handling (try, except, finally)

1) Why do we need exceptions?

Programs sometimes face surprises: a user types letters instead of a number, a file doesn't exist, or a number divides by zero.
An exception is Python's way of saying, "Something went wrong at runtime."
Without handling exceptions, your program may crash. With try/except/finally, you can catch problems and keep going smoothly.

2) The main ideas

try: Put code here that might cause an error.
except: If an error happens, Python jumps here so you can handle it.
finally: This always runs at the end, whether there was an error or not. It's great for cleanup (like closing files or printing a "Done" message).

3) Basic syntax

try:
    risky_code()
except SomeSpecificError:
    handle_that_error()
except AnotherError as e:
    print("Details:", e)
finally:
    always_runs()

Note:

Catch specific errors first. Use a generic except Exception as e only as a last fallback.
You can also use else: it runs only if no exception happened.

Easy and clean code examples

A) Converting input safely (ValueError)

print("Enter your age:")
try:
    age = int(input("> "))
    print("You are", age, "years old.")
except ValueError:
    print("Please enter numbers only.")
finally:
    print("Thanks for using the age checker!")

B) Dividing numbers (multiple except, with else and finally)

try:
    a = int(input("Enter the numerator: "))
    b = int(input("Enter the denominator: "))
    result = a / b
except ValueError:
    print("Only whole numbers are allowed.")
except ZeroDivisionError:
    print("You cannot divide by zero.")
else:
    print("Result:", result)
finally:
    print("Calculation attempt finished.")

C) Seeing the actual error message (as e)

items = ["apple", "banana", "cherry"]
try:
    print(items[5])  # This index doesn't exist
except IndexError as e:
    print("Oops! Index problem:", e)
finally:
    print("Index check complete.")

D) Cleanup with finally (closing a file)

file = None
try:
    file = open("data.txt", "r")
    content = file.read()
    print("File length:", len(content))
except FileNotFoundError:
    print("data.txt not found. Make sure the file exists next to your script.")
finally:
    if file is not None:
        file.close()
        print("File closed.")
    print("Done reading attempt.")

Note: In real projects, we often prefer with open(...) as f: because it auto-closes the file. finally is still useful for many other kinds of cleanup.

Mini Project: Safe Calculator (loop + try/except/finally)

Goal: Build a simple calculator that keeps running until the user quits. It handles wrong inputs and division by zero without crashing, and always prints a neat divider line in finally.

Steps:

Ask for two numbers.
Ask for an operation: +, -, *, /
Handle these errors:
- ValueError for non-numeric input
- ZeroDivisionError for dividing by zero
Use finally to print a divider every round.
Let the user type q to quit.

Code:

print("Welcome to SafeCalc! Type q at any prompt to quit.")

while True:
    try:
        left = input("Left number: ")
        if left.lower() == "q":
            break
        right = input("Right number: ")
        if right.lower() == "q":
            break
        op = input("Operation (+, -, *, /): ")
        if op.lower() == "q":
            break

        a = float(left)
        b = float(right)

        if op == "+":
            result = a + b
        elif op == "-":
            result = a - b
        elif op == "*":
            result = a * b
        elif op == "/":
            result = a / b  # may raise ZeroDivisionError if b == 0.0
        else:
            print("Unknown operation. Choose from +, -, *, /.")
            continue

        print("Answer:", result)

    except ValueError:
        print("Please enter valid numbers for left and right.")
    except ZeroDivisionError:
        print("Cannot divide by zero. Try a different denominator.")
    finally:
        print("-----")  # Always prints, even if there was an error

print("Goodbye from SafeCalc!")

Challenge ideas:

Add power (^) and modulo (%) operations.
Count how many successful calculations happened.
Log each attempt to a list; at the end, print the history.

Summary

Exceptions are runtime problems. Use try to attempt risky code.
except catches specific errors and lets your program respond, not crash.
finally always runs, perfect for cleanup or final messages.
Write specific except blocks first; only use a general except Exception as e at the end if needed.
Practice makes it natural—wrap tricky code, handle what can go wrong, and keep your programs friendly and reliable.

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