The 10 Biggest Python Mistakes Newbies Make

Avoid these common pitfalls and level up your Python skills!

🎥 Watch This Python Guide

🔔 Subscribe on YouTube

👉 Like 👍 | Comment 💬 | Subscribe 🔔 for more Python tips

Table of Contents

Mistake 1: Not Using Proper Indentation

Indentation is crucial in Python because it defines code blocks. Unlike other languages that use curly braces {}, Python relies on spaces. You should always use 4 spaces for indentation (PEP 8).

Wrong: 
score = 67
if score > 50:
  print('you pass') # 2 spaces (inconsistent)

score = 67
if score > 50:
    print('you pass') # 4 spaces
Correct: 
score = 67
if score > 50:
    print('you pass') # Consistent 4 spaces

Mistake 2: Forgetting to Close Files

Forgetting to close files can cause issues like data not being saved or resources being locked. Using the with statement is the best practice.

Wrong: 
f = open('dataset.txt', 'w')
f.write('new_data')
f.close() 
# If an error happens before close(), the file stays open!
Correct: 
with open('dataset.txt', 'w') as f:
    f.write('new_data')
# Automatically closes the file, even if errors occur.

Mistake 3: Confusing = with ==

Newbies often mix up the assignment operator (=) and the comparison operator (==).

Wrong: 
score = 67
if score = 50: # Syntax Error!
    print('You passed!')
Correct: 
score = 67
if score == 50: # Correct comparison
    print('You passed!')

Mistake 4: Not Understanding Scope & Lifetime

Scope defines where a variable is accessible. Variables defined inside a function are not accessible outside of it.

Wrong: 
def greet():
    message = "Hello!"

print(message) # Error: message is not defined globally
Correct: 
def greet():
    message = "Hello!"
    print(message) # Correct usage inside scope

Mistake 5: Using import *

Importing everything using * pollutes the namespace and can lead to conflicts. It is better to be explicit.

Wrong: 
from math import *
print(floor(2.4))
print(pi) # Where did these come from? Unclear.
Correct: 
import math
print(math.floor(2.4))
print(math.pi) # Clear and clean namespace

Mistake 6: Forgetting to Handle Exceptions

Handling exceptions prevents crashes and allows you to show friendly error messages.

Wrong: 
age = int(input("Enter your age: "))
print("Your age is:", age)
# Crashes if user enters "ten"
Correct: 
try:
    age = int(input("Enter your age: "))
    print("Your age is:", age)
except ValueError:
    print("Please enter a valid number.")

Mistake 7: Overusing Global Variables

Modifying global variables inside functions can lead to bugs. It's often better to pass arguments and return values.

Wrong: 
count = 0
def increment():
    count += 1 # UnboundLocalError
increment()
Better Practice: 
def calculate_sum():
    result = 0
    for i in range(1, 5):
        result += i
    return result # Use return values instead of globals

Mistake 8: Inefficient String Concatenation

Using + repeatedly in a loop is slow because strings are immutable. Use .join() instead.

Wrong: 
msg = ""
for name in names:
    msg += name + ", "
Correct: 
# Efficient concatenation
msg = ", ".join(names)

Mistake 9: Neglecting Documentation

Docstrings help others (and future you) understand what your functions do.

Wrong: 
def add(a, b):
    return a + b
Correct: 
def add(a, b):
    """Calculates the sum of two numbers."""
    return a + b

Mistake 10: Ignoring Virtual Environments

Installing libraries globally causes version conflicts. Always use a virtual environment for each project.

Wrong: 
pip install numpy # Installing globally
Correct: 
python -m venv myenv
source myenv/bin/activate
pip install numpy # Installing in isolated environment