Python
  • Understand what errors and exceptions are in Python
  • Recognize different types of errors
  • Read and interpret error messages
  • Understand the error traceback

Understanding Errors

Imagine you're following a recipe to bake a cake. What if the recipe says "add 2 cups of flurp"? You'd stop and say, "What's flurp? I can't continue!" That's exactly what happens in programming – when Python encounters something it doesn't understand or can't do, it raises an error.

Errors aren't your enemy – they're your friend! They tell you exactly what went wrong and where. Learning to read and understand errors is one of the most valuable skills a programmer can have. Let's demystify them!

Two Types of Problems

Python has two main categories of problems:


                    Types of Problems                             

                                                                  
   SYNTAX ERRORS (Compile-time)                                  
                                     
   • Python can't even understand your code                      
   • Caught BEFORE the program runs                               
   • Like grammar mistakes in a sentence                          
   • Example: missing colon, mismatched parentheses              
                                                                  
   EXCEPTIONS (Runtime)                                          
                                             
   • Valid syntax, but something goes wrong during execution     
   • Caught WHILE the program runs                                
   • Like trying to open a door that doesn't exist               
   • Example: dividing by zero, file not found

Syntax Errors

Syntax errors occur when Python can't parse your code:

#  Missing colon
if True
    print("Hello")
# SyntaxError: expected ':'

#  Mismatched parentheses
print("Hello"
# SyntaxError: unexpected EOF while parsing

#  Invalid syntax
x = 5 +
# SyntaxError: invalid syntax

#  Incorrect indentation
def greet():
print("Hello")
# IndentationError: expected an indented block

How Python Shows Syntax Errors

  File "script.py", line 2
    if True
          ^
SyntaxError: expected ':'

The arrow (^) points to where Python got confused!

Common Exceptions

Exceptions occur during program execution. Here are the most common ones:

TypeError - Wrong Type

# Trying to do something with incompatible types
result = "hello" + 5
# TypeError: can only concatenate str (not "int") to str

len(42)
# TypeError: object of type 'int' has no len()

ValueError - Wrong Value

# Right type, but wrong value
int("hello")
# ValueError: invalid literal for int() with base 10: 'hello'

int("12.5")  # Can't convert float string directly to int
# ValueError: invalid literal for int() with base 10: '12.5'

NameError - Unknown Name

# Using a variable that doesn't exist
print(undefined_variable)
# NameError: name 'undefined_variable' is not defined

IndexError - Bad Index

# Accessing an index that doesn't exist
my_list = [1, 2, 3]
print(my_list[10])
# IndexError: list index out of range

KeyError - Missing Key

# Accessing a dictionary key that doesn't exist
my_dict = {"name": "Alice"}
print(my_dict["age"])
# KeyError: 'age'

ZeroDivisionError - Division by Zero

# Mathematical impossibility
result = 10 / 0
# ZeroDivisionError: division by zero

FileNotFoundError - Missing File

# Trying to open a file that doesn't exist
with open("nonexistent.txt") as f:
    content = f.read()
# FileNotFoundError: [Errno 2] No such file or directory: 'nonexistent.txt'

AttributeError - Missing Attribute

# Accessing an attribute that doesn't exist
"hello".append("!")
# AttributeError: 'str' object has no attribute 'append'

Reading Error Messages

Error messages have a structure – learn to read them!


                    Anatomy of an Error                           

                                                                  
   Traceback (most recent call last):                            
     File "main.py", line 10, in <module>    ← WHERE (file/line) 
       result = calculate(x)                  ← THE CODE          
     File "main.py", line 5, in calculate     ← DEEPER CALL      
       return value / divisor                 ← THE CODE          
   ZeroDivisionError: division by zero        ← WHAT (error type)
                              ← WHY (message)    
                                                                  
   Read from BOTTOM to TOP:                                      
   1. Error type (ZeroDivisionError)                             
   2. Error message (division by zero)                           
   3. Where it happened (line numbers, files)

Real Example

def get_item(items, index):
    return items[index]

def process_data():
    my_list = [1, 2, 3]
    return get_item(my_list, 10)

result = process_data()

The traceback:

Traceback (most recent call last):
  File "example.py", line 8, in <module>
    result = process_data()
  File "example.py", line 6, in process_data
    return get_item(my_list, 10)
  File "example.py", line 2, in get_item
    return items[index]
IndexError: list index out of range

Reading it:

  1. Bottom: IndexError: list index out of range - we know WHAT happened
  2. Line 2: The actual line that failed - items[index]
  3. Trace up: We can see the call chain that led here

Common Error Patterns

Pattern 1: The Typo

# Did you mean 'print'?
prnt("Hello")
# NameError: name 'prnt' is not defined

# Variable name typo
user_name = "Alice"
print(username)  # Missing underscore!
# NameError: name 'username' is not defined

Pattern 2: The Off-by-One

# Lists are 0-indexed!
fruits = ["apple", "banana", "cherry"]
print(fruits[3])  # Only indices 0, 1, 2 exist
# IndexError: list index out of range

Pattern 3: The None Surprise

# Many functions return None
result = [1, 2, 3].sort()  # sort() modifies in place, returns None
print(result[0])
# TypeError: 'NoneType' object is not subscriptable

Pattern 4: The Type Mismatch

# User input is always a string!
age = input("Enter your age: ")  # Returns "25", not 25
next_year = age + 1
# TypeError: can only concatenate str (not "int") to str

Practical Tips

Tip 1: Read the Last Line First

# When you see an error, start at the bottom!
# The last line tells you WHAT happened
# Then trace up to find WHERE

Tip 2: Copy the Error, Search Online

# If you don't understand an error:
# 1. Copy the error type and message
# 2. Search: "Python ZeroDivisionError division by zero"
# 3. Stack Overflow usually has the answer!

Tip 3: Check the Line Number

# The traceback shows line numbers
# Go to that line in your code
# Often the bug is there or one line above

Key Takeaways


                   Remember These Points                          

                                                                  
   Syntax Errors: Python can't understand your code            
     Caught before running (missing colons, parentheses)          
                                                                  
   Exceptions: Code runs but something goes wrong              
     TypeError, ValueError, KeyError, IndexError, etc.            
                                                                  
   Read tracebacks from BOTTOM to TOP                          
     Error type → Message → Line numbers                          
                                                                  
   Error messages are helpful!                                 
     They tell you WHAT happened and WHERE                        
                                                                  
   When stuck, search the error message online

What's Next?

Now you can read errors! But how do you handle them gracefully so your program doesn't crash? In the next lesson, we'll learn try and except – Python's way of catching and handling errors.