Lab Questions

Pokemon Nesting Example

Exceptions

Failure

Code can fail for a variety of reasons
Some errors can be caught before the program is run
Others must be handled during runtime

Example

>>> speed = input("What is the air velocity of an unladen swallow?")
What is the air velocity of an unladen swallow?
What do you mean, an African or a European swallow?
>>> int(speed)
ValueError: invalid literal for int() with base 10:

Exceptions

Raised when error occurs during runtime
Immediately ends the program with an error message*

Example

num = input("Enter a number between 1 and 10")

square = int(num) * int(num)

print("The square of your number is:", square)

Catching Exceptions

It is possible to continue execution after an exception is raised
This requires us to catch the exception from a try block

Example

try:
    int("red")
except ValueError:
    print("Please enter a valid number")

Processing Exceptions

Report the error to the user
Try again
Do something else

Try again

entry = input("Enter a number between 1 and 10")

try:
    num = int(entry)
except ValueError:
    entry = input("That's not a number. Try again:")
    num = int(entry)

square = num * num
print("The square of your number is:", square)

Do something else

entry = input("Enter a number between 1 and 10")

try:
    num = int(entry)
except ValueError:
    print("That's not a number. Let's just use 5.")
    num = 5

square = num * num
print("The square of your number is:", square)

Catching all exceptions

By default, all exceptions are caught by except
This can cause problems
You generally want to avoid catching all exceptions

Example

dividend = input("Enter dividend:")
divisor = input("Enter divisor:")

try:
    print("The quotient is", int(dividend) / int(divisor))
except:
    print("You did not enter numbers.")

Ignoring Exceptions

We should almost never ignore exceptions
It is possible to ignore them using pass

Pass

entry = input("Enter a number between 1 and 10")

try:
    num = int(entry)
except ValueError:
    pass

square = num * num
print("The square of your number is:", square)

Pass keyword

The pass keyword is valid in many places
It is not explicitly related to exceptions
It simply indicates that while a statement is expected, we don’t have anything to do

Example

i = 1

if i == 2:
    pass
else:
    print("i is not 2")