Exception Handling

Three Types of Programming Error

You must be able to identify and distinguish between syntax, runtime and logic errors in the exam.

try / except / finally

Wrap potentially dangerous code in a try block. If an exception is raised, control jumps to the matching except (or catch) block. The finally block always runs regardless.

try:
    number = int(input("Enter a number: "))
    result = 100 / number
    print(f"Result: {result}")

except ValueError:
    print("Error: please enter a whole number.")

except ZeroDivisionError:
    print("Error: cannot divide by zero.")

finally:
    print("Calculation complete.")  # always runs

try
{
    Console.Write("Enter a number: ");
    int number = int.Parse(Console.ReadLine());
    double result = 100.0 / number;
    Console.WriteLine($"Result: {result}");
}
catch (FormatException)
{
    Console.WriteLine("Error: please enter a whole number.");
}
catch (DivideByZeroException)
{
    Console.WriteLine("Error: cannot divide by zero.");
}
finally
{
    Console.WriteLine("Calculation complete.");  // always runs
}

Raising Exceptions Deliberately

You can raise an exception yourself when data is invalid, rather than letting the program continue with bad values. This makes bugs easier to find and fixes easier to make.

def set_age(age):
    if age < 0 or age > 130:
        raise ValueError(f"Age {age} is not valid.")
    return age

try:
    set_age(-5)
except ValueError as e:
    print(e)  # Age -5 is not valid.

static int SetAge(int age)
{
    if (age < 0 || age > 130)
        throw new ArgumentOutOfRangeException(
            "age", $"Age {age} is not valid.");
    return age;
}

try { SetAge(-5); }
catch (ArgumentOutOfRangeException e)
    { Console.WriteLine(e.Message); }

Error Trigger Sandbox

Select a scenario to see how each exception type behaves - with and without a try/except block.

Three Quick Challenges

try:
    print(1 / 0)
except ZeroDivisionError:
    print("caught")
finally:
    print("done")

try { Console.WriteLine(1 / 0); }
catch (DivideByZeroException)
  { Console.WriteLine("caught"); }
finally
  { Console.WriteLine("done"); }

try:
    result = int("abc")
except:
    pass
print(result)

Check Your Understanding

Practice and Consolidation