exceptions

Handling Exceptions

Raising Exceptions

You can create errors with your own messages that stop the program:

:::python
raise ValueError("expected a valid Pokemon name")

Catching Exceptions

The try.. except clause allows your program to catch errors and act on them instead of terminating the program.

:::python
cards = "234567890JQKA"

for card in cards:
    try:
        number = int(card)
        print(f"dealt {card}")
    except ValueError:
        print(f"{card} could not be dealt")

How not to catch Exceptions

Exceptions should always be caught with an explicit error type. Using a generic except makes debugging difficult:

:::python
cards = "234567890JQKA"

for card in cards:
    try:
        number = int(card)
        print(f"dealt {car}")
    except:
        print(f"{card} could not be dealt")

In this example, car causes a NameError for every card. You don't get any clues what exactly went wrong.

This is called the "diaper pattern" and considered a very bad habit.

Creating your own Exceptions

You can define your own types of Exceptions:

:::python
class WrongInputError(Exception): pass

and raise them:

:::python
text = input("please enter a number between 1-4: ")
if not text in "1234":
    raise WrongInputError("{} is not a number between 1-4.".format(text))

The try..except works for your own Exception types as well.