Testing and Debugging in Python (`unittest` & `pdb`)

🟡 Intermediate

📖 Definition

Unit Testing: Automating the verification of individual isolated functions or modules using Python’s built-in unittest framework.
Debugging: Isolating, inspecting, and fixing runtime bugs and logical errors using breakpoint() / pdb (Python Debugger) or IDE debuggers.

🇮🇳 Hindi

Automated testing ke liye built-in unittest framework ka use hota hai. Code ko runtime par step-by-step debug karne ke liye Python 3.7+ mein breakpoint() function ka use kiya jata hai, jisse terminal par pdb interactive console open ho jata hai.

🚩 Marathi

Automated testing sathi unittest framework vaparatat. Code debug karnyasathi breakpoint() vapara.

📝 1. Built-in `unittest` Framework

import unittest

def add(a, b):
    return a + b

class TestMathOperations(unittest.TestCase):
    def test_add_positive_numbers(self):
        self.assertEqual(add(2, 3), 5)
        
    def test_add_negative_numbers(self):
        self.assertEqual(add(-1, -1), -2)

if __name__ == "__main__":
    unittest.main()

Key Assertions in `unittest.TestCase`:

self.assertEqual(a, b)
self.assertTrue(condition) / self.assertFalse(condition)
self.assertIsNone(obj) / self.assertIsNotNone(obj)
self.assertRaises(ExceptionType, func, *args)

📝 2. Programmatic Debugging with `breakpoint()` (Python 3.7+)

Placing breakpoint() pauses execution and opens the pdb (Python Debugger) interactive shell:

Key `pdb` Debugger Commands:

n (next): Executes current line and moves to next line.
s (step): Steps inside the called function.
c (continue): Resumes normal execution until next breakpoint.
p variable: Prints current variable value.
q (quit): Exits debugger.

def calculate_discount(price, percent):
    discount = price * (percent / 100)
    breakpoint() # Pauses execution here in terminal!
    return price - discount

💡 Complete Example: Function & Unit Test Module

import unittest

# 1. Code Module to Test
def divide(a, b):
    if b == 0:
        raise ZeroDivisionError("Cannot divide by zero.")
    return a / b

def is_palindrome(text):
    if not isinstance(text, str):
        raise TypeError("Input must be a string.")
    clean_text = "".join(c.lower() for c in text if c.isalnum())
    return clean_text == clean_text[::-1]

# 2. Unit Test Suite
class TestUtilityFunctions(unittest.TestCase):
    
    def test_valid_division(self):
        self.assertEqual(divide(10, 2), 5.0)
        self.assertAlmostEqual(divide(1, 3), 0.3333333, places=4)
        
    def test_division_by_zero_raises_exception(self):
        with self.assertRaises(ZeroDivisionError):
            divide(10, 0)
            
    def test_palindrome_valid(self):
        self.assertTrue(is_palindrome("A man, a plan, a canal: Panama"))
        self.assertFalse(is_palindrome("Python"))
        
    def test_palindrome_invalid_type(self):
        with self.assertRaises(TypeError):
            is_palindrome(12321)

# 3. Test Execution Runner
if __name__ == "__main__":
    print("=== RUNNING UNITTEST SUITE ===")
    unittest.main()

👀 Output

=== RUNNING UNITTEST SUITE ===
....
----------------------------------------------------------------------
Ran 4 tests in 0.002s

OK

⚠️ Common Mistakes

Writing unit tests that depend on execution order or external state (tests MUST remain independent!).
Leaving breakpoint() statements in production deployment code!
Asserting only happy-path inputs while ignoring edge cases and boundary conditions (e.g. None, 0, negative numbers).

🛡️ Safety / Important Notes

In modern Python development, pytest (pip install pytest) is widely used alongside unittest for cleaner test syntax and rich assertion failure diffs.

🌍 Real-World Usage

CI/CD deployment pipelines (GitHub Actions, Jenkins), regression testing during codebase refactoring, and debugging API issues.

🧪 Try It Yourself

Write a function square(x) and a unittest.TestCase verifying square(4) == 16.
Add a test asserting that square(-3) == 9.

🎯 Mini Challenge

Write a unit test for a BankAccount class verifying that withdrawing an amount greater than balance raises an InsufficientBalanceError.

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Testing and Debugging in Python (unittest & pdb)