Python Type Hints

Type hints let you describe what type of data a variable or function expects.

  Important:
  Python will still run without type hints. Type hints are mainly for:
  - clearer code
  - better autocomplete in editors
  - catching mistakes early with tools (like mypy)
  
  ## Basic type hints
  
  ```python
  def greet(name: str) -> str:
      return f"Hello {name}!"
  
  age: int = 25
  price: float = 19.99
  is_active: bool = True
  
  print(greet("Tom"))
  ```
  
  Expected output:
  
  ```
  Hello Tom!
  ```
  
  ## Collection hints (modern style)
  
  ```python
  from typing import List, Dict, Tuple
  
  names: List[str] = ["Tom", "Sarah", "Mike"]
  scores: Dict[str, int] = {"Tom": 90, "Sarah": 85}
  point: Tuple[int, int] = (10, 20)
  
  print(names)
  print(scores)
  print(point)
  ```
  
  ## Optional (value can be None)
  
  ```python
  from typing import Optional
  
  def find_user(user_id: int) -> Optional[str]:
      if user_id == 1:
          return "Tom"
      return None
  
  print(find_user(1))
  print(find_user(99))
  ```
  
  Expected output:
  
  ```
  Tom
  None
  ```
  
  ## Union (multiple allowed types)
  
  ```python
  from typing import Union
  
  def process(value: Union[int, str]) -> str:
      return str(value)
  
  print(process(123))
  print(process("abc"))
  ```
  
  Expected output:
  
  ```
  123
  abc
  ```
  
  ## Callable (function as a parameter)
  
  ```python
  from typing import Callable
  
  def apply(func: Callable[[int, int], int], x: int, y: int) -> int:
      return func(x, y)
  
  def add(a: int, b: int) -> int:
      return a + b
  
  print(apply(add, 5, 3))
  ```
  
  Expected output:
  
  ```
  8
  ```
  
  ## Graph: what type hints change
  
  ```mermaid
  flowchart LR
    A[Write code] --> B[Add type hints]
    B --> C[Editor autocomplete improves]
    B --> D[Static checks catch mistakes]
    B --> E[Code becomes clearer]
  ```
  
  Type hints do not replace tests, but they make refactoring safer and reduce simple mistakes.
  
  In the next lesson, you will learn threading basics, including when to use threads and when not to.