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 REACHEDexceptIndexError:print("OOPS - AN ERROR")print("BOTTOM")
TOP
OOPS - AN ERROR
BOTTOM
print("TOP")try:100/0# triggers a ZeroDivisionErrorprint("EVERYTHING IS GOING FINE") # NEVER REACHEDexceptZeroDivisionError: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 codeexceptExceptionas err:print(type(err)) #> this will tell you the error typeprint(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 notin options:raiseValueError("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):passraise MyCustomError("My custom message")
16.3 Common Errors and Explanations
16.3.1 Syntax Error
A SyntaxError occurs when the code is not written correctly according to the rules of the Python language. This can happen due to missing colons, parentheses, or incorrect indentation.
Example:
# Missing colonifTrueprint("This will cause a SyntaxError")
16.3.2 Value Error
A ValueError occurs when a function receives an argument of the correct type but an inappropriate value. This can happen when trying to convert a string to an integer, but the string does not represent a number.
Example:
# Trying to convert a non-numeric string to an integerint("hello") # This will cause a ValueError
16.3.3 Key Error
A KeyError occurs when trying to access a dictionary with a key that does not exist. This can happen when trying to retrieve a value from a dictionary using a key that is not present.
Example:
# Accessing a non-existent key in a dictionarymy_dict = {"name": "Alice"}print(my_dict["age"]) # This will cause a KeyError
16.3.4 Index Error
An IndexError occurs when trying to access an index that is out of range for a list or other sequence. This can happen when trying to access an element at an index that does not exist.
Example:
# Accessing an index that is out of rangemy_list = [1, 2, 3]print(my_list[5]) # This will cause an IndexError