1. Introduction to Errors and Exceptions

Quiz: 0/5

An error happens when something goes wrong in your code, stopping Python from running it properly. Think of it like a red traffic light—Python sees a problem and says, “Stop! I don’t know what to do.”

Errors can be frustrating, but they’re actually helpful! Python gives you messages to explain what went wrong, so you can fix the problem.

Types of Errors

1. Syntax Errors

These errors happen when Python doesn’t understand your code because it breaks the rules of the language. It’s like writing a sentence with bad grammar.

Example:

print("Hello"  

Error: SyntaxError: unexpected EOF while parsing
Fix: Add the missing closing parenthesis ) like this:

print("Hello")

2. Runtime Errors

These happen when Python understands your code but runs into a problem while executing it.

Example:

number = 10 / 0

Error: ZeroDivisionError: division by zero
Fix: You can’t divide by zero! Change the denominator to a nonzero number.

Common Built-in Exceptions

Python has special error types (called exceptions) that tell you exactly what went wrong. Here are some common ones:

Exception	When It Happens	Example
TypeError	When you use the wrong type of data	5 + "hello" (You can't add a number and a string!)
ValueError	When a function gets the wrong kind of value	int("abc") (You can't turn letters into a number!)
IndexError	When you try to access an item that doesn’t exist in a list	my_list = [1, 2, 3] → my_list[5] (There’s no 6th item!)

Reading and interpreting Error messages

When Python throws an error, it gives you a traceback—a message showing what went wrong. Let’s look at one:

Example:

numbers = [1, 2, 3]
print(numbers[5])  

Error Message:

Traceback (most recent call last):
  File "script.py", line 2, in <module>
    print(numbers[5])
IndexError: list index out of range

How to read this?

1. "File 'script.py', line 2" → The error happened on line 2.
2. "print(numbers[5])" → This is the line causing the error.
3. "IndexError: list index out of range" → The problem: there’s no item at index 5 in the list!

Fix: Make sure you only access valid indexes:

print(numbers[2])  # Works fine! (index 2 exists)

Key takeaways

• Errors are normal! They help you debug and improve your code.
• Syntax errors happen when Python doesn’t understand your code.
• Runtime errors happen when something goes wrong while running the code.
• Read error messages carefully—they tell you where and what the problem is.