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        content="Chapter 17 of Python Zero to Hero: learn how to use type hints, annotations and the typing module for static typing."/>
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        content="Python, typing, type hints, annotations, mypy, List, Dict, Optional, Union, Any, TypeVar, Generic"/>
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  <title>Chapter 17 – Type Hints & Static Typing | Python Zero to Hero</title>

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    <header class="my-4">
      <h1 class="display-6">Chapter 17: Type Hints &amp; Static Typing</h1>
      <p class="text-muted">
        Add type annotations to functions, variables and classes, and leverage the typing module for safer, self-documenting code.
      </p>
      <a href="src/chapter17.py" download class="btn btn-outline-primary btn-sm btn-py">
        Download <code>chapter17.py</code>
      </a>
      <hr>
    </header>

    <section id="objectives" class="mb-5">
      <h2>Objectives</h2>
      <ul>
        <li>Annotate function parameters and return types.</li>
        <li>Use variable annotations.</li>
        <li>Import and apply common types from <code>typing</code> (List, Dict, Optional, Union, Any).</li>
        <li>Define generics with <code>TypeVar</code> and <code>Generic</code>.</li>
        <li>Run static type checks with <code>mypy</code>.</li>
      </ul>
    </section>

    <section id="func-annotations" class="mb-5">
      <h2>1. Function Annotations</h2>
      <pre><code class="python">def greet(name: str) -> str:
    return f"Hello, {name}"

def add(a: int, b: int) -> int:
    return a + b

print(greet.__annotations__)
# {'name': <class 'str'>, 'return': <class 'str'>}
</code></pre>
    </section>

    <section id="var-annotations" class="mb-5">
      <h2>2. Variable Annotations</h2>
      <pre><code class="python">from typing import List

ages: List[int] = [25, 30, 35]
threshold: float = 4.5

print(globals().get('__annotations__'))
# {'ages': list[int], 'threshold': float}
</code></pre>
    </section>

    <section id="typing" class="mb-5">
      <h2>3. Common Types from <code>typing</code></h2>
      <pre><code class="python">from typing import Optional, Union, Any, Dict

def find_user(user_id: int) -> Optional[Dict[str, Any]]:
    users = {1: {"name": "Alice"}}
    return users.get(user_id)

def process(data: Union[str, bytes]) -> None:
    print(data)

user = find_user(2)
if user is None:
    print("Not found")
</code></pre>
    </section>

    <section id="generics" class="mb-5">
      <h2>4. Generics &amp; TypeVar</h2>
      <pre><code class="python">from typing import TypeVar, Generic, List

T = TypeVar('T')

class Stack(Generic[T]):
    def __init__(self) -> None:
        self._items: List[T] = []

    def push(self, item: T) -> None:
        self._items.append(item)

    def pop(self) -> T:
        return self._items.pop()

int_stack = Stack[int]()
int_stack.push(1)
str_stack = Stack[str]()
str_stack.push("hello")
</code></pre>
    </section>

    <section id="mypy" class="mb-5">
      <h2>5. Static Type Checking with <code>mypy</code></h2>
      <pre><code class="bash">pip install mypy
mypy src/chapter17.py --strict
</code></pre>
      <p>Fix reported mismatches to ensure annotations align with usage.</p>
    </section>

    <section id="exercises" class="mb-5">
      <h2>Exercises</h2>
      <ol>
        <li>Add type hints to a small calculator module (add, sub, mul, div).</li>
        <li>Annotate a data-loading function returning <code>List[Dict[str, Any]]</code>.</li>
        <li>Create a generic <code>Queue[T]</code> class with <code>enqueue()</code> and <code>dequeue()</code>.</li>
        <li>Run <code>mypy</code> on your code and resolve all type errors.</li>
      </ol>
    </section>

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