PEP 484: Type Hints - Static Type Checking for Python

def greet(name: str, age: int) -> str:
    return f"Hello {name}, you are {age} years old"

# Variables can be annotated too
count: int = 0
message: str = "Hello, World!"
is_active: bool = True

# Function with no return value
def log_message(message: str) -> None:
    print(f"LOG: {message}")

from typing import List, Dict, Set, Tuple, Optional

# Collection types
numbers: List[int] = [1, 2, 3, 4]
user_data: Dict[str, int] = {"age": 30, "score": 95}
tags: Set[str] = {"python", "typing", "pep484"}
coordinates: Tuple[float, float] = (3.14, 2.71)

# Optional types (can be None)
def find_user(user_id: int) -> Optional[str]:
    users = {1: "Alice", 2: "Bob"}
    return users.get(user_id)  # Returns str or None

from typing import Union, Generic, TypeVar

# Union types - can be one of several types
def process_id(user_id: Union[int, str]) -> str:
    return str(user_id)

# Generic types
T = TypeVar('T')

def first_item(items: List[T]) -> Optional[T]:
    return items[0] if items else None

# Usage preserves type information
first_number = first_item([1, 2, 3])  # Type: Optional[int]
first_word = first_item(["a", "b"])   # Type: Optional[str]

from typing import ClassVar
from datetime import datetime

class User:
    # Class variable
    total_users: ClassVar[int] = 0
    
    def __init__(self, name: str, email: str) -> None:
        self.name: str = name
        self.email: str = email
        self.created_at: datetime = datetime.now()
        User.total_users += 1
    
    def get_info(self) -> Dict[str, str]:
        return {
            "name": self.name,
            "email": self.email
        }
    
    @classmethod
    def create_admin(cls, name: str) -> 'User':
        return cls(name, f"{name}@admin.com")

from typing import Callable, Protocol, Literal

# Function types
def apply_operation(x: int, operation: Callable[[int], int]) -> int:
    return operation(x)

# Usage
result = apply_operation(5, lambda x: x * 2)

# Protocols (structural typing)
class Drawable(Protocol):
    def draw(self) -> None: ...

def render(obj: Drawable) -> None:
    obj.draw()  # Any object with a draw() method works

# Literal types
def set_mode(mode: Literal["debug", "production"]) -> None:
    print(f"Setting mode to {mode}")

set_mode("debug")      # OK
set_mode("testing")    # Type checker error

# Install mypy
pip install mypy

# Check your code
mypy your_script.py

# Example with type errors
def add_numbers(a: int, b: int) -> int:
    return a + b

result = add_numbers("5", "10")  # mypy error: incompatible types