Error Handling
==============

Error handling is a way of dealing with problems that happen when a program runs.

Sometimes, programs crash because of mistakes like typing the wrong input. Instead of stopping the program completely, error handling lets the program catch these problems and respond safely.

Example 1 - Handling Errors When Adding Numbers
-----------------------------------------------

.. code-block:: python
    :linenos:

    try:
        numberOne = float(input("Enter a number: "))
        numberTwo = float(input("Enter another number: "))

        result = numberOne + numberTwo

        print(f"Result: {result}")

    except ValueError:
        print("You must enter numbers only.")

.. dropdown:: Show Output

    | Enter a number: 5
    | Enter another number: ten
    | You must enter numbers only.

    .. admonition:: Explanation

        This program asks the user to type in two numbers. It then adds the two numbers together and shows the result on the screen.

        The ``try`` block contains the code that might cause an error - in this case, when the user enters something that isn’t a number. If that happens, Python will stop running the try block and jump to the ``except`` block instead.

        The ``except ValueError:`` block then displays the message "You must enter numbers only." This prevents the program from crashing and gives the user clear feedback about what went wrong.

Example 2 - Using a While Loop to Validate User Input
-----------------------------------------------------

.. code-block:: python
    :linenos:

    numberInvalid = True

    while numberInvalid == True:
        try:
            number = int(input("Enter a number: "))
            numberInvalid = False

        except ValueError:
            print("Please enter a number.")

.. dropdown:: Show Output

    | Enter a number: ten
    | Please enter a number.
    | Enter a number: 1a
    | Please enter a number.
    | Enter a number: 10

    .. admonition:: Explanation

        At the start of the program, the variable ``numberInvalid`` is set to ``True``. This means that the program assumes the user has not yet entered a valid number.

        The ``while`` loop will continue running as long as ``numberInvalid`` is ``True``. Inside the loop, the program tries to run the code inside the ``try`` block. It asks the user to enter a value using the ``input()`` function and then attempts to convert that input into an integer using ``int()``.

        If the user enters something that can be converted into a number, such as ``5`` or ``12``, the conversion will succeed. The program then sets ``numberInvalid`` to ``False``, which causes the loop to stop.

        However, if the user enters something that cannot be turned into a number, such as ``cat`` or ``hello``, the program will produce a ``ValueError``. When this happens, the ``except`` block runs and displays the message ``"Please enter a number."`` on the screen. The loop then starts again, asking the user to try again.