AssertionError in Python — Causes, Examples, and Fixes

 What is an Assertion in Python?

An assertion is a debugging statement used to verify conditions during program execution.

Syntax:

assert condition, "Optional error message"

• If the condition is True, the program continues normally.

• If the condition is False, Python raises an AssertionError.

---

Simple Example of AssertionError

x = 10
assert x > 20

Output:

AssertionError

Because 10 is not greater than 20.

---

Example with Custom Error Message

age = 16
assert age >= 18, "Age must be at least 18"

Output:

AssertionError: Age must be at least 18

---

Why Does AssertionError Occur?

Here are the most common causes:

1️⃣ False Condition

assert 5 == 10

2️⃣ Wrong Logic in Code

marks = 30
assert marks >= 35

3️⃣ Unexpected Input Value

number = -5
assert number > 0

4️⃣ Missing or None Value

name = None
assert name is not None

---

Real-World Example

Checking Password Length

password = "abc123"
assert len(password) >= 8, "Password too short"

---

AssertionError vs Exception

AssertionError	Exception
Used for debugging	Used for runtime errors
Helps test assumptions	Handles real program failures
Mostly for developers	For real user input errors
Can be disabled	Always active

---

How to Fix AssertionError

✅ Correct the Condition Logic

Wrong:

assert age > 18

Correct:

assert age >= 18

---

✅ Check Input Before Assertion

if age is not None:
    assert age >= 18

---

Important Note: Avoid Assertions in Production

Assertions can be disabled by running:

python -O script.py

So they should not be used for user input validation or critical application logic.

---

Best Practices for Using Assertions

✔ Debugging and testing
✔ Checking internal logic
✔ Validating assumptions in development
❌ Not for production error handling
❌ Not for validating external user input

---

Beginner Tip

Assertions answer this question:

“Is my code behaving the way I expect it to?”

If not, Python alerts you immediately.