Debugging Code in Python: A Complete Guide to Fixing Common Errors

Python is one of the most versatile programming languages, but no matter how experienced you are, errors are unavoidable. The real skill lies in debugging code in Python efficiently—understanding what went wrong and applying the right fix.

In this guide, we’ll explore what debugging in Python means, review the most common errors, and show you practical ways to debug Python code like a professional.

1. SyntaxError – The Typo Problem

Occurs when Python cannot interpret your code due to formatting mistakes.

print("Hello World')   # mismatched quotes

print("Hello World')   # mismatched quotes

    print("Hello World')
          ^
SyntaxError: unterminated string literal (detected at line 1)

✅ Fix: Use matching quotes and check for missing colons.

print(“Hello World”)

2. IndentationError – Whitespace Gone Wrong

Python depends on indentation to structure blocks. Misaligned spaces or tabs lead to issues.

def greet():
print("Hello")  # not indented

def greet():
print("Hello")  # not indented

print("Hello")  # not indented
    ^
IndentationError: expected an indented block after function definition on line 1

✅ Fix: Stick to 4 spaces consistently.

def greet():
print(“Hello”)

3. NameError – Undefined Variables

Happens when trying to use a variable that doesn’t exist.

print(username)

print(username)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
----> 1 print(username)

NameError: name 'username' is not defined

✅ Fix: Define variables before use and check spelling.

username=’Python’
print(username)

4. TypeError – Mixing Incompatible Types

Occurs when incompatible data types are combined.

age = 25
print("Age: " + age)

age = 25
print("Age: " + age)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
      1 age = 25
----> 2 print("Age: " + age)

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

✅ Fix: Convert explicitly with str(), int(), or float().

age = 25
print(“Age: ” + str(age))

5. IndexError – Out of Range

Triggered when accessing an invalid index in a list.

colors = ["red", "blue", "green"]
print(colors[3])

colors = ["red", "blue", "green"]
print(colors[3])

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
      1 colors = ["red", "blue", "green"]
----> 2 print(colors[3])

IndexError: list index out of range

✅ Fix: Use len(list) to stay within range. In python list index starts from 0 and ends with length -1

colors = [“red”, “blue”, “green”]
print(colors[2])

6. KeyError – Dictionary Lookup Fails

Raised when a dictionary key doesn’t exist.

student = {"name": "Alice"}
print(student["age"])

student = {"name": "Alice"}
print(student["age"])

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
      1 student = {"name": "Alice"}
----> 2 print(student["age"])

KeyError: 'age'

✅ Fix: Use .get(key, default) for safe lookups.

student = {“name”: “Alice”}
student.get(‘name’)
print(student[“name”])

7. AttributeError – Wrong Method or Attribute

Occurs when calling a method that doesn’t exist for an object.

text = "hello"
text.push("world")

text = "hello"
text.push("world")

✅ Fix: Use correct string methods like .find() or .replace(). You can write the string then “.” and pres “Tab” to know all the function from Jupyter notebook.

8. ValueError – Invalid Input

Happens when a valid type but invalid value is passed.

int("ten")

int("ten")

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
----> 1 int('ten')

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

✅ Fix: Validate input before conversion or wrap in try/except.

try:
    num = int("ten")
except ValueError:
    num = 0

try:
    num = int("ten")
except ValueError:
    num = 0

9. ZeroDivisionError – Dividing by Zero

Python doesn’t allow division by zero—it’s mathematically undefined. Occurs in financial or scientific calculations when denominators are unexpectedly 0.

x = 5 / 0

x = 5 / 0

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
----> 1 x=5/0

ZeroDivisionError: division by zero

✅ Fix: Always check denominator before dividing.

denominator = 0
if denominator != 0:
    print(5 / denominator)
else:
    print("Cannot divide by zero")

denominator = 0
if denominator != 0:
    print(5 / denominator)
else:
    print("Cannot divide by zero")

10. ImportError / ModuleNotFoundError – Missing Libraries

Triggered when modules are missing or misspelled. Occurs when Python can’t find the module you’re trying to import.

import numppy

import numppy

---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
----> 1 import numppy

ModuleNotFoundError: No module named 'numppy'

✅ Fix: Install with pip install numpy and check spelling.

pip install numpy

Pro Debugging Tips

To truly debug Python code like an expert:

• Use print() for quick checks.
• Apply logging for structured debugging.
• Use IDEs like VS Code or PyCharm for breakpoints.
• Write unit tests with pytest to prevent repeat issues.

Final Thoughts

So, what is debugging in Python? It’s the process of identifying and fixing errors so your code runs smoothly. You need to debug and correct the error befording doing the testing. From SyntaxError to ZeroDivisionError, the key is learning how to debug the Python code systematically.

The next time you face an error, don’t panic. Treat it as a guide toward writing more reliable, efficient, and professional code.