Reading Python Errors

What You'll Learn

How to read Python error messages (tracebacks), recognize common error types, and fix them confidently.

Don't Fear the Traceback

A traceback is Python telling you exactly what went wrong and where. Most beginners panic when they see one — don't. It's free information.

Traceback (most recent call last):
  File "script.py", line 10, in main
    result = divide(x, y)
  File "script.py", line 5, in divide
    return x / y
ZeroDivisionError: division by zero

Read it bottom to top:

1. Bottom line — the actual error type and message: ZeroDivisionError: division by zero
2. Line above — the exact code that failed: return x / y
3. Lines above that — the call stack showing how you got there

Common Error Types

SyntaxError — Code isn't valid Python

print("hello"    # Missing closing )

SyntaxError: '(' was never closed

Fix: Check for unclosed brackets, missing colons, or mismatched quotes.

---

NameError — Variable or function doesn't exist

print(messge)    # typo: "messge" instead of "message"

NameError: name 'messge' is not defined

Fix: Check spelling, check if the variable was actually defined, check scope.

---

TypeError — Wrong type of value

age = 25
print("Age: " + age)    # can't add str and int

TypeError: can only concatenate str (not "int") to str

Fix: Cast to the right type: print("Age: " + str(age))

---

IndexError — List index out of range

items = ["a", "b", "c"]
print(items[5])    # only indexes 0, 1, 2 exist

IndexError: list index out of range

Fix: Check list length with len(items) before accessing by index.

---

KeyError — Dictionary key doesn't exist

user = {"name": "Alice"}
print(user["email"])    # "email" key doesn't exist

KeyError: 'email'

Fix: Use .get() with a default: user.get("email", "not set")

---

AttributeError — Object doesn't have that method or attribute

name = "alice"
print(name.uppercase())    # method is "upper()", not "uppercase()"

AttributeError: 'str' object has no attribute 'uppercase'

Fix: Check the correct method name in the docs or with dir(name).

---

FileNotFoundError — File doesn't exist at that path

with open("data.txt") as f:
    content = f.read()

FileNotFoundError: [Errno 2] No such file or directory: 'data.txt'

Fix: Check the path is correct: ls data.txt in your terminal.

---

ValueError — Right type, wrong value

number = int("hello")    # "hello" can't be converted to int

ValueError: invalid literal for int() with base 10: 'hello'

Fix: Validate before converting, or use a try/except.

---

IndentationError — Wrong indentation

if True:
print("hello")    # not indented

IndentationError: expected an indented block after 'if' statement

Fix: Add 4 spaces of indentation.

How to Debug Systematically

When your script fails, follow this order:

1. Read the bottom of the traceback first
         ↓
2. Find the exact line number and file
         ↓
3. Read the code around that line
         ↓
4. Add print() to inspect variable values
         ↓
5. Fix the root cause (not just the symptom)

Add print() to Inspect State

def calculate_average(numbers):
    print(f"DEBUG: numbers={numbers}, type={type(numbers)}")  # ← add this
    total = sum(numbers)
    return total / len(numbers)

Remove the debug prints once fixed.

Using Python's Built-in Debugger (pdb)

import pdb

def risky_function(data):
    pdb.set_trace()   # execution stops here, you can inspect variables
    result = data["key"]
    return result

In pdb:

• n — next line
• p variable — print variable value
• q — quit

Or use breakpoint() (Python 3.7+) instead of import pdb; pdb.set_trace().

Quick Error Reference

Error	Common Cause	Quick Fix
SyntaxError	Typo, missing :, unclosed bracket	Read the line carefully
NameError	Typo, variable not defined yet	Check spelling and scope
TypeError	Wrong type (str + int, etc.)	Cast to correct type
IndexError	List index too large	Check len() first
KeyError	Dict key missing	Use .get(key, default)
AttributeError	Wrong method name	Use dir() or check docs
FileNotFoundError	Wrong path	Check with ls or Path.exists()
ValueError	Right type, wrong value	Validate before converting
IndentationError	Wrong spacing	Use 4 spaces, never tabs
ZeroDivisionError	Dividing by 0	Check divisor before dividing

Practice: Fix These Errors

Try to predict the error before running each one:

# 1.
print("The answer is " + 42)

# 2.
fruits = ["apple", "banana"]
print(fruits[2])

# 3.
data = {"name": "Alice"}
print(data["age"])

# 4.
if True:
print("hello")

# 5.
number = int("3.14")

What's Next

→ Module 2: Language Basics