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<!DOCTYPE html>
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<head>
  <meta charset="UTF-8"/>
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  <meta name="description"
        content="Chapter 9 of Python Zero to Hero: learn error handling, raising exceptions, custom exceptions, and using assertions."/>
  <meta name="keywords"
        content="Python, errors, exceptions, try, except, else, finally, raise, custom exception, assertion"/>
  <meta name="author" content="Luca Bocaletto"/>
  <title>Chapter 9 – Errors & Exceptions | Python Zero to Hero</title>

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    <header class="my-4">
      <h1 class="display-6">Chapter 9: Errors & Exceptions</h1>
      <p class="text-muted">
        Handle runtime errors gracefully, raise your own exceptions, and use assertions for debugging.
      </p>
      <a href="src/chapter9.py" download class="btn btn-outline-primary btn-sm btn-py">
        Download <code>chapter9.py</code>
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    </header>

    <!-- Objectives -->
    <section id="objectives" class="mb-5">
      <h2>Objectives</h2>
      <ul>
        <li>Distinguish errors vs exceptions in Python.</li>
        <li>Use <code>try</code>/<code>except</code> to catch exceptions.</li>
        <li>Leverage <code>else</code> and <code>finally</code> clauses.</li>
        <li>Raise built-in exceptions with <code>raise</code>.</li>
        <li>Define and use custom exception classes.</li>
        <li>Employ <code>assert</code> for sanity checks.</li>
      </ul>
    </section>

    <!-- 1. Errors vs Exceptions -->
    <section id="errors-vs-exceptions" class="mb-5">
      <h2>1. Errors vs Exceptions</h2>
      <p>
        An error is a problem in your code (syntax, indentation) detected before execution. An exception
        is a runtime event that can be caught and handled.
      </p>
      <pre><code class="python"># SyntaxError: missing parenthesis
print "Hello"

# ZeroDivisionError at runtime
print(1/0)</code></pre>
    </section>

    <!-- 2. try/except -->
    <section id="try-except" class="mb-5">
      <h2>2. The try/except Block</h2>
      <p>
        Wrap code that may fail in <code>try</code>, catch specific exceptions in <code>except</code>.
      </p>
      <pre><code class="python">try:
    result = 10 / int(input("Enter divisor: "))
    print("Result:", result)
except ValueError:
    print("Please enter a valid integer.")
except ZeroDivisionError:
    print("Cannot divide by zero.")</code></pre>
    </section>

    <!-- 3. else & finally -->
    <section id="else-finally" class="mb-5">
      <h2>3. else & finally</h2>
      <p>
        <code>else</code> runs if no exception occurs; <code>finally</code> always runs.
      </p>
      <pre><code class="python">try:
    f = open("data.txt")
    data = f.read()
except FileNotFoundError:
    print("Missing file.")
else:
    print("Read data:", data[:50])
finally:
    f.close()
    print("File closed.")</code></pre>
    </section>

    <!-- 4. Raising Exceptions -->
    <section id="raising" class="mb-5">
      <h2>4. Raising Exceptions</h2>
      <p>
        Use <code>raise</code> to trigger an exception manually.
      </p>
      <pre><code class="python">def check_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    return True

# call
check_age(-5)  # ValueError: Age cannot be negative</code></pre>
    </section>

    <!-- 5. Custom Exceptions -->
    <section id="custom-exceptions" class="mb-5">
      <h2>5. Custom Exception Classes</h2>
      <p>
        Subclass <code>Exception</code> to define your own.
      </p>
      <pre><code class="python">class InvalidConfigurationError(Exception):
    """Raised when configuration is invalid."""
    pass

def load_config(cfg):
    if "host" not in cfg:
        raise InvalidConfigurationError("Missing host setting")</code></pre>
    <p>You can catch it like any built-in exception:</p>
      <pre><code class="python">try:
    load_config({})
except InvalidConfigurationError as e:
    print("Config error:", e)</code></pre>
    </section>

    <!-- 6. Assertions -->
    <section id="assertions" class="mb-5">
      <h2>6. Assertions</h2>
      <p>
        Use <code>assert</code> for internal sanity checks. They can be disabled with <code>-O</code>.
      </p>
      <pre><code class="python">def factorial(n):
    assert n >= 0, "n must be non-negative"
    return 1 if n in (0,1) else n * factorial(n-1)

factorial(-1)  # AssertionError: n must be non-negative</code></pre>
    </section>

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      <h2>Exercises</h2>
      <ol>
        <li>Write a function that reads an integer and retries until a valid int is entered.</li>
        <li>Create a custom exception <code>AuthenticationError</code> and use it in a login function.</li>
        <li>Implement file copy with <code>try/finally</code> to ensure files are closed.</li>
        <li>Add <code>assert</code> statements to validate function inputs for a calculator module.</li>
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