day-10

Exception Handling

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While coding, you will occasionally encounter some errors that will appear once you execute your own program, and it's quite frustrating to see it almost every time you click run. For the Day 10, we will learn about the errors and exceptions, and how we can handle it in our program.

Errors and Exceptions

Error messages such as Syntax Error: invalid syntax or IndentationError: expected an indented block after function definition on line x may have already appeared on your terminal when you first start writing code in Python, and these are called the Errors.

Some errors you may encounter in Python includes:

Error	Description
AttributeError	when an attribute assignment or reference fails
KeyError	when a key is not found in a dictionary
NameError	when a variable is not found in the local or global scope
KeyboardInterrupt	when the user hits the interrupt key (Ctrl+C or Del)
SyntaxError	when a syntax error is encountered by the parser
TabError	when the indentation consists of inconsistent tabs and spaces
TypeError	when a function or operation is applied to an object of an incorrect type
ValueError	when a function gets an argument of correct type but improper value
ZeroDivisionError	when the second operand of a division or module operation is zero (0)

But if we detected these errors and prevent them from executing the particular code that will raise an error, it is now called an exception.

Exceptions are basically errors that are detected and handled during execution. If an error is not detected, it will result in an error message that we have mentioned earlier.

How do we detect an error?

age = input("Please enter your age: ")

if (age > 18):
    print("You are not eligible for a discount.")
else:
    print("You are eligible.")

If we try to run the code above, it will cause an error if the user inputs a non-digit character:

TypeError: '>' not supported between instances of 'str' and 'int'

This is because the program only assumes that the input will enter a valid type, and didn't take into account the illegal inputs which can cause fatal errors in our program.

In Python, we can handle exceptions using try ... except ... statement like this:

age = input("Please enter your age: ")

try: 
    if (age > 18):
        print("You are not eligible for a discount.")
    else:
        print("You are eligible.")
except TypeError:
    print("Invalid input.")

In the code snippet above, the program will first execute the code inside the try block.

• If no exceptions occurs in the try block, then the except block is skipped and the entire try block will be executed.
• If an exception occurs, then the rest of the code inside the try block is skipped, and if the error matches the exception named after the except keyword, then the except block is executed.
• If the error didn't match the exception named after the except keyword, then it is considered to be an unhandled exception and an error message will be displayed after the execution stops.

We can also have different except block for different exceptions:

try: 
    if (age > 18):
        print("You are not eligible for a discount.")
    else:
        print("You are eligible.")
except TypeError:
    print("Invalid input.")
except ValueError:
    print("Invalid value.")

or have many exceptions in a single block:

try: 
    if (age > 18):
        print("You are not eligible for a discount.")
    else:
        print("You are eligible.")
except (TypeError, ValueError):
    print("Invalid input.")

It also has an optional else block that is executed if no exceptions occured.

def divide(a, b):
    return a / b

try:
    a = int(input("Enter a number: "))
    b = int(input("Enter a second number: "))
    quotient = divide(a, b)
except Exception as e:
    print(f"Invalid input: {e}")
else:
    print(f"The quotient is {quotient}")

The quotient will be displayed if no errors are encountered in the try block.

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Additional Resources:

• Errors and Exceptions
• Python - Error Types
• Python Exception Handling