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<!DOCTYPE html>
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  <meta name="description"
        content="Chapter 12 of Python Zero to Hero: learn debugging with pdb, logging best practices, and basic unit testing with unittest."/>
  <meta name="keywords"
        content="Python, debugging, pdb, logging, unittest, test, logging levels, handlers, assertions"/>
  <meta name="author" content="Luca Bocaletto"/>
  <title>Chapter 12 – Debugging, Logging & Testing | Python Zero to Hero</title>

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    <header class="my-4">
      <h1 class="display-6">Chapter 12: Debugging, Logging &amp; Testing</h1>
      <p class="text-muted">
        Techniques to find and fix bugs, record events with logging, and write automated tests.
      </p>
      <a href="src/chapter12.py" download class="btn btn-outline-primary btn-sm btn-py">
        Download <code>chapter12.py</code>
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    <!-- Objectives -->
    <section id="objectives" class="mb-5">
      <h2>Objectives</h2>
      <ul>
        <li>Use print() and <code>pdb</code> for interactive debugging.</li>
        <li>Configure the <code>logging</code> module: levels, formatters, handlers.</li>
        <li>Differentiate <code>assert</code> statements from exceptions and logs.</li>
        <li>Write basic unit tests with <code>unittest</code> and run them.</li>
        <li>Learn about third-party testing tools like <code>pytest</code>.</li>
      </ul>
    </section>

    <!-- 1. Debugging -->
    <section id="debugging" class="mb-5">
      <h2>1. Debugging Techniques</h2>
      <p>Start with strategic <code>print()</code> calls:</p>
      <pre><code class="python">def compute(x, y):
    print("DEBUG:", x, y)
    return x / y
compute(5, 0)  # inspect inputs</code></pre>
      <p>Use the built-in <code>pdb</code> for breakpoints:</p>
      <pre><code class="python">import pdb

def buggy():
    a = 1
    pdb.set_trace()    # pause here
    b = 0
    return a / b

buggy()</code></pre>
    </section>

    <!-- 2. Logging -->
    <section id="logging" class="mb-5">
      <h2>2. Logging Module</h2>
      <p>A better alternative to prints; supports levels and destinations:</p>
      <pre><code class="python">import logging

logging.basicConfig(
    level=logging.DEBUG,
    format="%(asctime)s [%(levelname)s] %(message)s",
    datefmt="%H:%M:%S"
)
logger = logging.getLogger(__name__)

logger.debug("Debugging info")
logger.info("Startup complete")
logger.warning("Low disk space")
logger.error("An error occurred")</code></pre>
      <p>Create file or stream handlers for flexible output:</p>
      <pre><code class="python">fh = logging.FileHandler("app.log")
fh.setLevel(logging.WARNING)
logger.addHandler(fh)</code></pre>
    </section>

    <!-- 3. Assertions -->
    <section id="assertions" class="mb-5">
      <h2>3. Assertions</h2>
      <p>Use <code>assert</code> to check invariants during development:</p>
      <pre><code class="python">def divide(a, b):
    assert b != 0, "b must not be zero"
    return a / b</code></pre>
      <p>Note: <code>assert</code> statements are removed when Python runs with <code>-O</code>.</p>
    </section>

    <!-- 4. Unit Testing -->
    <section id="unittest" class="mb-5">
      <h2>4. Unit Testing with unittest</h2>
      <p>Create test cases by subclassing <code>unittest.TestCase</code>:</p>
      <pre><code class="python">import unittest
from chapter12 import divide

class TestMath(unittest.TestCase):
    def test_divide_normal(self):
        self.assertEqual(divide(10, 2), 5)

    def test_divide_zero(self):
        with self.assertRaises(AssertionError):
            divide(5, 0)

if __name__ == "__main__":
    unittest.main()</code></pre>
      <p>Run tests with <code>python -m unittest</code> or integrate into your CI.</p>
    </section>

    <!-- 5. pytest & Beyond -->
    <section id="pytest" class="mb-5">
      <h2>5. pytest Overview</h2>
      <p><code>pytest</code> offers concise syntax and fixtures:</p>
      <pre><code class="python"># test_chapter12.py
import pytest
from chapter12 import divide

def test_divide():
    assert divide(9, 3) == 3

def test_divide_zero():
    with pytest.raises(AssertionError):
        divide(1, 0)</code></pre>
      <p>Install with <code>pip install pytest</code> and run <code>pytest</code>.</p>
    </section>

    <!-- Exercises -->
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      <h2>Exercises</h2>
      <ol>
        <li>Insert <code>pdb.set_trace()</code> into a small function and step through it.</li>
        <li>Switch <code>print()</code> calls to <code>logging</code> at various levels and inspect the log file.</li>
        <li>Add <code>assert</code> checks to a data-validation function.</li>
        <li>Write <code>unittest</code> tests for a simple calculator module (add, sub, mul, div).</li>
        <li>Try rewriting those tests in <code>pytest</code> style and compare.</li>
      </ol>
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