16  Error Handling

Sometimes our programs will encounter errors.

16.1 Handling Errors

In Python, we can use a try... except block to handle these errors.

After running into an error for the first time, we should observe what type of error we are experiencing (e.g. KeyError, IndexError, ZeroDivisionError, etc.).

Once we know what type of error we need to handle, we should wrap the problematic code inside the try clause, and specify the known error type in the except clause:

print("TOP")

try:
    empty_list = []
    matching_item = empty_list[0] # triggers IndexError (list index out of range)
    print("EVERYTHING IS GOING FINE") # NEVER REACHED
except IndexError:
    print("OOPS - AN ERROR")

print("BOTTOM")

TOP
OOPS - AN ERROR
BOTTOM

print("TOP")

try:
    100 / 0 # triggers a ZeroDivisionError
    print("EVERYTHING IS GOING FINE") # NEVER REACHED
except ZeroDivisionError:
    print("OOPS - AN ERROR")

print("BOTTOM")

TOP
OOPS - AN ERROR
BOTTOM

If we’re not yet sure what type of error we’re experiencing, we can temporarily catch all error classes that inherit from the base error class (Exception), and once caught, we print the specific error’s datatype to learn how to handle it:

try:
    do_something() # some hypothetical problematic code
except Exception as err:
    print(type(err)) #> this will tell you the error type
    print(err) #> the error message

16.2 Raising Errors

If we find the need to trigger our own errors to stop program execution (less common), we can use the raise keyword followed by the type of error (e.g. ValueError):

options = ["rock", "paper", "scissors"]

choice = "hoya" # input("Please choose either 'rock', 'paper', or 'scissors': ")

if choice not in options:
    raise ValueError("OOPS - Please type 'rock', or 'paper', or 'scissors'.")

16.2.1 Defining and Raising Custom Errors

We can define our own errors if that’s helpful, by inheriting a class from the base Exception class (or preferably a more specific one):

class MyCustomError(Exception):
   pass

raise MyCustomError("My custom message")