Common errors

Errors you’ll encounter

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

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

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

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", "[email protected]")

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

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")

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))

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.

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:

Error type: ZeroDivisionError
Error message: division by zero
Location: script.py, line 5
Code that failed: result = 10 / 0

Common mistake patterns

Off-by-one errors

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

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

Modifying while iterating

# 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]

Mutable default arguments

# 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

What’s next?

Now that you have completed all error handling concepts, test and reinforce your understanding with hands-on practice problems.

Practice & Exercises

Practice catching and raising errors

Functions

Classes

Error handling

Modules & Packages

Extending Python

Errors you’ll encounter

FileNotFoundError

ValueError

KeyError

IndexError

TypeError

AttributeError

Reading error messages

Common mistake patterns

What’s next?

Practice & Exercises

​Errors you’ll encounter

​FileNotFoundError

​ValueError

​KeyError

​IndexError

​TypeError

​AttributeError

​Reading error messages

​Common mistake patterns

​What’s next?

Practice & Exercises

Errors you’ll encounter

FileNotFoundError

ValueError

KeyError

IndexError

TypeError

AttributeError

Reading error messages

Common mistake patterns

What’s next?