Module 13: Error Handling

In this module, we will learn about Error Handling. We will learn how to handle unexpected crashes (errors/exceptions) in our programs so that the code runs smoothly without stopping.

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13.1 What are Errors?

Errors are mistakes in code. In Python, errors are grouped into two main categories:

1. Syntax Errors: Mistakes in writing code rules. These prevent the program from running at all.
2. Runtime Errors (Exceptions): Mistakes that happen while the program is running, even if the syntax is correct.

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13.2 Syntax Errors

These happen when you violate the grammar rules of Python. The code cannot execute until you fix these.

# if age >= 18 # Missing the colon (:) at the end
#     print("Vote")

Error message: SyntaxError: expected ':'

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13.3 Runtime Errors (Exceptions)

The code syntax is correct, but something goes wrong during execution based on user inputs or system states.

Examples:

• ZeroDivisionError: Trying to divide a number by zero.

# result = 10 / 0 # Throws ZeroDivisionError

• ValueError: Passing an invalid value for conversion.

# num = int("hello") # Throws ValueError (cannot convert text to integer)

• FileNotFoundError: Trying to read a file that does not exist.

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13.4 Using try and except

We use the try-except block to catch and handle runtime errors so that our program does not crash.

• try Block: Put the code that might cause an error here.
• except Block: Put the code to run if an error happens here.

try:
    number = int(input("Enter a number: "))
    result = 100 / number
    print("Result is:", result)
except ZeroDivisionError:
    print("Error: You cannot divide by zero!")
except ValueError:
    print("Error: Please enter a valid number!")
except Exception as e:
    print("An unexpected error occurred:", e)

Now, if the user inputs 0 or text, the program prints a helpful error message instead of crashing.

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13.5 The finally Block

The finally block runs no matter what. It executes whether an error occurs or not. It is typically used for cleanup tasks like closing open files or database connections.

try:
    print("Opening database...")
    x = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")
finally:
    print("Closing database connection... (Always Runs)")

Output:

Opening database...
Cannot divide by zero!
Closing database connection... (Always Runs)

:::tip Golden Rule Never trust user inputs. Always use try-except blocks to protect your programs from crashing due to unexpected input values! :::