Understanding Errors and Exceptions
Overview
Errors occur when something goes wrong, such as dividing by zero or accessing a missing file. You'll understand the difference between syntax and runtime errors and how exceptions help Python handle unexpected situations, making your programs more robust.
What You Will Learn in This Lesson
By the end of this lesson, you will know:
- What errors are: Understand when and why errors occur in Python.
- Types of errors: Learn about syntax errors vs runtime errors (exceptions).
- Common exceptions: Recognize common error types like FileNotFoundError, ValueError, etc.
- Error messages: Learn to read and understand Python error messages.
- Why exceptions exist: Understand how exceptions help make programs more robust.
What Are Errors?
An error occurs when something goes wrong in your program. Python stops executing and shows you an error message explaining what happened.
result = 10 / 0
# ZeroDivisionError: division by zeroWhy Errors Happen
- Invalid operations (like dividing by zero)
- Missing files or resources
- Wrong data types
- Typos or syntax mistakes
- Accessing things that don't exist
Two Types of Errors
Python has two main categories of errors:
Syntax Errors
Mistakes in your code's grammar. Python can't even run the program.
# Missing colon
if x == 5
print(x)
# SyntaxError: expected ':'Runtime Errors (Exceptions)
Errors that happen while the program is running. The code syntax is correct, but something goes wrong.
# Code is correct, but file doesn't exist
file = open("missing.txt", "r")
# FileNotFoundError: [Errno 2] No such fileKey Difference
Syntax errors prevent your program from running at all. Exceptions occur while your program is running and can be handled.
Common Exceptions
Here are some common exceptions you'll encounter:
| Exception | When It Occurs | Example |
|---|---|---|
FileNotFoundError |
File doesn't exist | open("missing.txt", "r") |
ValueError |
Wrong value type | int("abc") |
ZeroDivisionError |
Dividing by zero | 10 / 0 |
IndexError |
List index out of range | my_list[10] (list has 5 items) |
KeyError |
Dictionary key doesn't exist | my_dict["missing"] |
TypeError |
Wrong data type for operation | "hello" + 5 |
Reading Error Messages
Python error messages tell you exactly what went wrong. Let's learn to read them:
Traceback (most recent call last):
File "example.py", line 3, in <module>
result = 10 / 0
ZeroDivisionError: division by zeroUnderstanding the Message
- Traceback: Shows where the error occurred
- File and line: Tells you the file name and line number
- Error type: The type of exception (ZeroDivisionError)
- Error message: Description of what went wrong
File-Related Exceptions
When working with files, you'll encounter specific exceptions:
file = open("nonexistent.txt", "r")
# FileNotFoundError: [Errno 2] No such file or directory: 'nonexistent.txt'# Trying to write to a read-only file
file = open("/readonly/file.txt", "w")
# PermissionError: [Errno 13] Permission deniedNote: These exceptions can be handled with try and except blocks, which we'll learn in the next lesson!
Practice: Understanding Errors
Try It YourselfTry creating different types of errors to see what happens:
Try it: Uncomment different lines to see different error messages. Notice how Python tells you exactly what went wrong!
Understanding Tracebacks
A traceback shows the sequence of function calls that led to an error:
Traceback (most recent call last):
File "example.py", line 10, in <module>
result = divide_numbers(10, 0)
File "example.py", line 5, in divide_numbers
return a / b
ZeroDivisionError: division by zeroReading Tracebacks
To read a traceback effectively:
- Start at the bottom: The last line shows the exception type and message
- Read upwards: Each line shows where the code was called from
- Find your code: Look for your own files (not Python's internal files)
- Check line numbers: The line number tells you exactly where the error occurred
def read_config():
return open("config.txt", "r").read()
def process_config():
config = read_config()
return config.upper()
def main():
result = process_config()
print(result)
main()
# If config.txt doesn't exist, traceback shows:
# - main() called process_config()
# - process_config() called read_config()
# - read_config() tried to open file → FileNotFoundErrorWhy Exceptions Are Useful
Exceptions might seem annoying, but they're actually helpful:
Early Detection
They catch problems immediately, before they cause worse issues. Better to fail fast than produce incorrect results!
Clear Messages
They tell you exactly what went wrong and where. The traceback points you to the exact line of code.
Handling
You can catch and handle exceptions to make your program more robust. Programs don't have to crash!
Debugging
Exceptions help you find bugs quickly. The error message and traceback give you clues about what went wrong.
Think of Exceptions as Safety Nets
Exceptions prevent your program from continuing with bad data or invalid operations. They force you to handle edge cases and make your code more reliable.
Summary
In this lesson, you learned:
- Errors: Occur when something goes wrong in your program
- Syntax errors: Prevent the program from running (grammar mistakes)
- Exceptions: Runtime errors that occur while the program runs
- Common exceptions: FileNotFoundError, ValueError, ZeroDivisionError, etc.
- Error messages: Tell you what went wrong and where
- Useful: Exceptions help catch problems early and can be handled
Next Step
In the next lesson, you'll learn how to handle exceptions gracefully using try and except blocks!
End-of-Lesson Exercises
Think about these questions to reinforce what you've learned:
Exercise 1: Error Types
What's the difference between a syntax error and an exception? Give an example of each.
Exercise 2: Common Exceptions
Name three common exceptions you might encounter when working with files, and explain when each occurs.