Python Notes
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Master Python error handling by understanding the most common errors — SyntaxError, IndentationError, NameError, TypeError, AttributeError, ImportError, ValueError, KeyError, IndexError, and how to fix them.
Every Python developer encounters errors. Learning to read error messages and understand common pitfalls is one of the most valuable skills you can develop.
Reading a Traceback
Code example
Traceback (most recent call last):
File "app.py", line 15, in <module> ← Script, line number, context
result = divide(10, 0) ← Code that caused the chain
File "app.py", line 8, in divide ← Function where error occurred
return a / b ← Exact line
ZeroDivisionError: division by zero ← Exception type: messageReading strategy: Always read the last line first (the actual error), then trace upward through the call stack.
SyntaxError
Example
# ❌ SyntaxError: Missing colon
def greet(name)
print(f"Hello, {name}")
# ✅ Fix
def greet(name):
print(f"Hello, {name}")
# ❌ SyntaxError: Unmatched bracket
result = (1 + 2 * 3
value = result + 1 # Forgot to close parenthesis
# ✅ Fix
result = (1 + 2 * 3)
value = result + 1
# ❌ SyntaxError: Missing quote
message = "Hello World
print(message)
# ✅ Fix
message = "Hello World"
print(message)
# ❌ SyntaxError: Invalid assignment target
3 = x # Can't assign to a literal
# ✅ Fix
x = 3
# ❌ f-string with backslash (Python < 3.12)
# names = ['Alice', 'Bob']
# print(f"Names: {'\n'.join(names)}") # SyntaxError!
# ✅ Fix — assign first or use Python 3.12+
separator = '\n'
names = ['Alice', 'Bob']
print(f"Names: {separator.join(names)}")
IndentationError
python example
# ❌ IndentationError: Expected an indented block
def greet(name):
print(f"Hello, {name}") # Not indented!
# ✅ Fix
def greet(name):
print(f"Hello, {name}")
# ❌ IndentationError: Unexpected indent
x = 5
y = 10 # Indented but not inside a block
# ✅ Fix
x = 5
y = 10
# ❌ Mixing tabs and spaces
def calculate():
x = 1
y = 2 # Tab instead of spaces!
# ✅ Fix — use spaces consistently
def calculate():
x = 1
y = 2
# Pro tip: Configure your editor to show whitespace and convert tabs to spacesNameError
python example
# ❌ NameError: name 'x' is not defined
print(x) # x was never assigned
# ✅ Fix
x = 10
print(x)
# ❌ NameError: typo in variable name
user_name = "Alice"
print(usernmae) # Typo!
# ✅ Fix
print(user_name)
# ❌ NameError: using variable before assignment
def calculate():
result = initial + 5 # initial not yet defined
initial = 10
return result
# ✅ Fix
def calculate():
initial = 10
result = initial + 5
return result
# ❌ NameError: function not imported
response = requests.get("https://example.com") # requests not imported
# ✅ Fix
import requests
response = requests.get("https://example.com")
# ❌ Scope issue
def outer():
x = 10
def inner():
print(x + 5) # OK — can read from outer scope
x = 20 # UnboundLocalError! Can't assign to x from outer scope
# ✅ Fix using nonlocal
def outer():
x = 10
def inner():
nonlocal x
x = 20
inner()
print(x) # 20TypeError
Example
# ❌ TypeError: can only concatenate str (not "int") to str
name = "Alice"
age = 25
print("Name: " + name + ", Age: " + age) # age is int!
# ✅ Fix
print("Name: " + name + ", Age: " + str(age))
print(f"Name: {name}, Age: {age}") # Best approach
# ❌ TypeError: unsupported operand type(s)
result = "5" + 3 # Can't add str and int
# ✅ Fix
result = int("5") + 3 # 8
result = "5" + str(3) # "53"
# ❌ TypeError: object is not subscriptable
number = 42
print(number[0]) # Integers aren't subscriptable
# ✅ Fix — check type first
data = "hello"
print(data[0]) # 'h'
# ❌ TypeError: ... takes N positional arguments but M were given
def add(a, b):
return a + b
result = add(1, 2, 3) # 3 args but function expects 2
# ✅ Fix
result = add(1, 2) # or add(*args)
# ❌ TypeError: 'NoneType' object is not callable
def get_function():
pass # Returns None!
func = get_function()
func() # None is not callable!
# ✅ Fix
def get_function():
return lambda x: x * 2 # Return an actual function
# ❌ TypeError: 'int' object is not iterable
for i in 5: # Can't iterate over integer
print(i)
# ✅ Fix
for i in range(5):
print(i)
AttributeError
python example
# ❌ AttributeError: 'str' object has no attribute 'append'
name = "Alice"
name.append(" Smith") # append() is for lists!
# ✅ Fix
name = name + " Smith" # or
name = f"{name} Smith"
# ❌ AttributeError: 'NoneType' object has no attribute 'upper'
result = some_function() # Returns None
print(result.upper()) # None has no methods!
# ✅ Fix — check for None
result = some_function()
if result is not None:
print(result.upper())
# Or: print(result.upper() if result else "No result")
# ❌ AttributeError: module has no attribute
import math
math.sqroot(16) # It's math.sqrt, not sqroot!
# ✅ Fix
math.sqrt(16)
# Common misspellings
# list.lenght → list.length (wrong!) → len(list) ✅
# dict.has_key() → 'key' in dict ✅ (Python 3)
# string.count() ← this one IS correct
# pd.DataFrame.tostring() → pd.DataFrame.to_string() ✅ImportError / ModuleNotFoundError
python example
# ❌ ModuleNotFoundError: No module named 'numpy'
import numpy as np # Module not installed
# ✅ Fix — install it
# pip install numpy
import numpy as np
# ❌ ImportError: cannot import name 'DataFrame' from 'pandas'
from pandas import DataFrame # This actually works...
# But this won't:
from pandas import Dataframe # Case sensitive!
# ✅ Fix — check case
from pandas import DataFrame # Capital D and F
# ❌ Circular imports
# file a.py:
# from b import B
# file b.py:
# from a import A # Circular!
# ✅ Fix — restructure or use local imports
# In a.py:
def use_b():
from b import B # Import inside function, not at module level
return B()
# ❌ Wrong relative import
from .models import User # This only works inside a package!
# ✅ Fix — use absolute import when running as script
from mypackage.models import UserValueError
Example
# ❌ ValueError: invalid literal for int() with base 10
age = int("twenty-five") # Can't convert text to int
# ✅ Fix — validate input
raw = "twenty-five"
try:
age = int(raw)
except ValueError:
print(f"'{raw}' is not a valid integer")
age = 0
# ❌ ValueError: not enough values to unpack
a, b, c = [1, 2] # Only 2 values, need 3
# ✅ Fix
a, b = [1, 2] # Match number of variables to values
# Or:
a, b, c = [1, 2, 3]
# ❌ ValueError in Pandas
import pandas as pd
df = pd.DataFrame({'a': [1, 2], 'b': [3, 4]})
df['c'] = [5, 6, 7] # Series length doesn't match!
# ✅ Fix — matching lengths
df['c'] = [5, 6]
# ❌ ValueError: math domain error
import math
math.sqrt(-1) # No real sqrt of -1
# ✅ Fix
import cmath
result = cmath.sqrt(-1) # Complex result: 1j
KeyError
Example
# ❌ KeyError: 'age'
person = {"name": "Alice", "city": "NYC"}
print(person["age"]) # Key doesn't exist!
# ✅ Fix — use .get() with default
print(person.get("age", 0)) # Returns 0 if key missing
print(person.get("name")) # Returns None if key missing
# Or check first
if "age" in person:
print(person["age"])
# ❌ KeyError in nested dict
data = {"user": {"name": "Alice"}}
print(data["user"]["email"]) # Missing key!
# ✅ Fix — chained .get()
email = data.get("user", {}).get("email", "N/A")
print(email) # "N/A"
# ❌ KeyError when modifying dict during iteration
d = {"a": 1, "b": 2, "c": 3}
for key in d:
if d[key] < 2:
del d[key] # RuntimeError: dictionary changed size!
# ✅ Fix — iterate over a copy
for key in list(d.keys()):
if d[key] < 2:
del d[key]
IndexError
Example
# ❌ IndexError: list index out of range
my_list = [10, 20, 30]
print(my_list[3]) # Index 3 doesn't exist (0, 1, 2)
print(my_list[-4]) # Negative index too large
# ✅ Fix — check length
if len(my_list) > 3:
print(my_list[3])
print(my_list[-1]) # Last element: safe
# ❌ IndexError from off-by-one
nums = [1, 2, 3, 4, 5]
for i in range(len(nums) + 1): # Should be len(nums)
print(nums[i]) # Fails on last iteration
# ✅ Fix
for i in range(len(nums)):
print(nums[i])
# Better: iterate directly
for num in nums:
print(num)
# ❌ IndexError on empty sequence
my_list = []
first = my_list[0] # Empty list!
# ✅ Fix — check first
if my_list:
first = my_list[0]
else:
first = None
# Or: first = my_list[0] if my_list else None
FileNotFoundError
python example
# ❌ FileNotFoundError: No such file or directory
with open("data.csv") as f:
content = f.read()
# ✅ Fix — check if file exists
from pathlib import Path
file_path = Path("data.csv")
if file_path.exists():
with open(file_path) as f:
content = f.read()
else:
print(f"File not found: {file_path}")
# Or use try/except
try:
with open("data.csv") as f:
content = f.read()
except FileNotFoundError:
print("data.csv not found!")
content = ""Exception Handling Best Practices
Example
# ❌ Catching all exceptions (too broad)
try:
result = int(user_input)
except:
print("Error") # Hides the real problem!
# ❌ Still too broad
try:
result = int(user_input)
except Exception:
print("Error")
# ✅ Catch specific exceptions
try:
result = int(user_input)
except ValueError:
print("Please enter a valid number")
except TypeError:
print("Input must be a string or number")
# ✅ Multiple exception types
try:
file_data = json.load(open(path))
value = file_data["key"]
except FileNotFoundError:
print(f"File not found: {path}")
except json.JSONDecodeError:
print("Invalid JSON format")
except KeyError:
print("Missing required key")
except Exception as e:
# Catch-all for unexpected errors (log, don't hide)
logger.error(f"Unexpected error: {type(e).__name__}: {e}")
raise # Re-raise to propagate
# ✅ finally block — always runs
def process_file(path):
f = None
try:
f = open(path)
data = json.load(f)
return data
except FileNotFoundError:
return None
finally:
if f:
f.close() # Runs whether exception occurred or not
# ✅ Better: context manager (auto-closes)
def process_file(path):
try:
with open(path) as f:
return json.load(f)
except FileNotFoundError:
return None
# ✅ Custom exceptions
class InsufficientFundsError(ValueError):
def __init__(self, balance, amount):
self.balance = balance
self.amount = amount
super().__init__(f"Insufficient funds: balance ${balance:.2f}, need ${amount:.2f}")
def withdraw(balance, amount):
if amount > balance:
raise InsufficientFundsError(balance, amount)
return balance - amount
try:
new_balance = withdraw(100, 200)
except InsufficientFundsError as e:
print(f"Transaction failed: {e}")
print(f" Balance: ${e.balance}, Requested: ${e.amount}")
Quick Error Reference
| Error | Typical Cause | Quick Fix |
|---|---|---|
SyntaxError | Missing colon, bracket, quote | Check syntax carefully |
IndentationError | Wrong indentation | Use 4 spaces consistently |
NameError | Variable not defined | Check spelling, define before use |
TypeError | Wrong type in operation | Convert types, check .get() for None |
AttributeError | Wrong method/attribute name | Check docs, look for typos |
ImportError | Module not installed | pip install module_name |
ValueError | Wrong value for operation | Validate input |
KeyError | Key missing from dict | Use .get() with default |
IndexError | Index out of range | Check len(), use negative indexing |
FileNotFoundError | File doesn't exist | Check path, use Path.exists() |
ZeroDivisionError | Division by zero | Check divisor before dividing |
RecursionError | Infinite recursion | Add base case, increase sys.setrecursionlimit |
*Next: Debugging Techniques →*
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