Common Python Errors

Common Python Errors#

Don’t Panic When You See Red#

Here’s a secret: professional programmers see errors constantly. The difference between a beginner and an expert isn’t that experts write perfect code. It’s that experts can read the error message and fix the problem in 30 seconds.

When you see a wall of red text, don’t panic. Python is trying to help you. It’s saying: “Something went wrong, and here’s exactly where and why.”

This page shows you the most common errors you’ll encounter. Learn to recognize them, and you’ll debug code 10× faster.

TypeError#

# Wrong: mixing types
result = "5" + 3  # TypeError: can only concatenate str to str

# Fix: convert types
result = int("5") + 3  # 8

IndentationError#

This is the most common beginner error. Python uses indentation (spaces at the start of lines) to define code blocks. Mixing tabs and spaces, or inconsistent indentation, causes this error.

# Wrong: inconsistent indentation
if x > 0:
print("positive")  # IndentationError: expected an indented block

# Wrong: mixed tabs and spaces
if x > 0:
    print("line 1")  # 4 spaces
	print("line 2")  # 1 tab - IndentationError!

# Correct: consistent 4-space indentation
if x > 0:
    print("positive")
    print("and non-zero")

Tip

Configure your editor to insert 4 spaces when you press Tab. In VS Code: Settings → “Editor: Tab Size” = 4 and “Editor: Insert Spaces” = checked.

NameError#

# Wrong: variable not defined
print(temperatrue)  # NameError: name 'temperatrue' is not defined

# Fix: check spelling
temperature = 300
print(temperature)

KeyError#

# Wrong: key doesn't exist
data = {"temp": 300}
print(data["pressure"])  # KeyError: 'pressure'

# Fix: use .get() with default
print(data.get("pressure", "Not found"))

FileNotFoundError#

# Wrong: file doesn't exist
f = open("data.txt")  # FileNotFoundError

# Fix: check path or handle error
try:
    f = open("data.txt")
except FileNotFoundError:
    print("File not found!")

How to Read Error Messages#

Traceback (most recent call last):
  File "script.py", line 5, in <module>    ← WHERE (file, line)
    result = x / 0
             ~~^~~
ZeroDivisionError: division by zero         ← WHAT went wrong

Always read from bottom up!

Debugging a Simulation: A Common Mistake#

Here’s an error that won’t show a red message but will give wrong results. Can you spot the bug?

# Euler's method for falling parachutist
g, c, m, dt = 9.8, 12.5, 68.1, 2.0
v = 0
t = 0

while t < 12:
    v = v + (g - c/m * v) * dt
    # Oops! Forgot to update t
    print(f"t = {t}, v = {v:.2f}")

This loop runs forever because t never changes! The fix:

while t < 12:
    v = v + (g - c/m * v) * dt
    t = t + dt  # Don't forget this!
    print(f"t = {t}, v = {v:.2f}")

Lesson: Not all bugs cause error messages. If your loop runs forever or gives unexpected results, check that you’re updating your loop variables.

Quick Reference#

Error

Common Cause

Fix

SyntaxError

Typo, missing colon, unmatched quotes

Check the line Python points to

NameError

Variable not defined or misspelled

Check spelling, define before use

TypeError

Wrong type (e.g., "5" + 3)

Convert types: int(), float(), str()

ValueError

Right type, wrong value (e.g., int("hello"))

Validate input before converting

KeyError

Dictionary key doesn’t exist

Use .get(key, default)

IndexError

List index out of range

Check len(list) before accessing

FileNotFoundError

File path is wrong

Check path, use try/except

ZeroDivisionError

Dividing by zero

Check denominator before dividing

Reading errors: Start from the bottom line (the error type), then look at the line number above it.

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