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Errors you’ll encounter

Here are the most common Python errors and how to fix them. Understanding these will save you hours of debugging.

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FileNotFoundError

Happens when a file doesn’t exist: 
# This fails if file doesn't exist
with open('missing.txt', 'r') as f:
    content = f.read()
Fix: 
import os

# Check first
if os.path.exists('data.txt'):
    with open('data.txt', 'r') as f:
        content = f.read()
else:
    print("File not found")

# Or use try-except
try:
    with open('data.txt', 'r') as f:
        content = f.read()
except FileNotFoundError:
    content = ""  # Default value

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ValueError

Happens when a value is wrong type or format: 
# These cause ValueError
int("hello")        # Can't convert to number
int("12.5")         # int() doesn't accept decimals
list.remove("x")    # Item not in list
Fix: 
# Validate user input
try:
    age = int(input("Age: "))
except ValueError:
    print("Please enter a number")

# Convert carefully
float_str = "12.5"
number = int(float(float_str))  # Convert to float first

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KeyError

Happens when a dictionary key doesn’t exist: 
user = {"name": "Alice", "age": 25}
print(user["email"])  # KeyError - no email key
Fix: 
# Check if key exists
if "email" in user:
    print(user["email"])

# Use get() with default
email = user.get("email", "no-email@example.com")

# Or handle the error
try:
    print(user["email"])
except KeyError:
    print("Email not found")

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IndexError

Happens when accessing invalid list position: 
numbers = [1, 2, 3]
print(numbers[5])  # IndexError - only 3 items
Fix: 
# Check length first
if len(numbers) > 5:
    print(numbers[5])

# Use negative indexing carefully
last_item = numbers[-1] if numbers else None

# Handle the error
try:
    print(numbers[5])
except IndexError:
    print("List too short")

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TypeError

Happens when operations use wrong types: 
# These cause TypeError
"hello" + 5            # Can't add string and number
len(42)                # Numbers don't have length
int([1, 2, 3])         # Can't convert list to int
Fix: 
# Convert types explicitly
"hello" + str(5)       # "hello5"
str(42)                # "42"

# Check type first
if isinstance(value, str):
    print(len(value))

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AttributeError

Happens when accessing non-existent attributes: 
text = "hello"
text.append("!")  # AttributeError - strings don't have append
Fix: 
# Use correct methods
text = "hello"
text = text + "!"  # Strings are immutable

# Check if attribute exists
if hasattr(obj, 'append'):
    obj.append(item)
Error messages are your friends! They tell you:
  • What went wrong (error type)
  • Where it happened (file and line number)
  • Why it happened (error message)
Always read the full error message before trying to fix it.

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Reading error messages

Python error messages show a “traceback”: 
Traceback (most recent call last):
  File "script.py", line 5, in <module>
    result = 10 / 0
ZeroDivisionError: division by zero
Read from bottom to top:
  1. Error type: ZeroDivisionError
  2. Error message: division by zero
  3. Location: script.py, line 5
  4. Code that failed: result = 10 / 0

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Common mistake patterns

# Wrong - lists start at 0
items = ["a", "b", "c"]
third_item = items[3]  # IndexError

# Right
third_item = items[2]  # "c"

# Wrong - changes list size during iteration
numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num % 2 == 0:
        numbers.remove(num)  # Dangerous!

# Right - create new list
odd_numbers = [n for n in numbers if n % 2 != 0]

# Wrong - same list for all calls!
def add_item(item, items=[]):
    items.append(item)
    return items

# Right - create new list each time
def add_item(item, items=None):
    if items is None:
        items = []
    items.append(item)
    return items

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What’s next?

Now that you understand error handling, let’s learn about classes - Python’s way of organizing code and data together.

Classes

Object-oriented programming