Python Type Hints — Static Typing for Python

Type hints document expected types and enable static analysis. They catch bugs before runtime and improve code readability.

Learning Objectives

• Add type hints to functions and variables
• Use advanced types (Union, Optional, TypeVar)
• Define protocols and typed structures
• Run mypy for static type checking

Basic Type Hints

def greet(name: str) -> str:
    return f"Hello, {name}"

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

# Variables
age: int = 25
name: str = "Alice"
scores: list[int] = [95, 87, 91]

Advanced Types

from typing import Union, Optional, TypeVar, Generic

# Union — multiple types
def process(value: Union[int, str]) -> str:
    return str(value)

# Optional — value or None
def find(name: str) -> Optional[dict]:
    return None

# TypeVar — generic types
T = TypeVar('T')

def first(items: list[T]) -> T:
    return items[0]

# TypedDict
from typing import TypedDict

class UserDict(TypedDict):
    name: str
    age: int
    email: str

# Callable
from typing import Callable

def apply(func: Callable[[int, int], int], a: int, b: int) -> int:
    return func(a, b)

Protocol (Structural Subtyping)

from typing import Protocol

class Drawable(Protocol):
    def draw(self) -> str: ...

class Circle:
    def draw(self) -> str:
        return "O"

class Square:
    def draw(self) -> str:
        return "[]"

def render(shape: Drawable) -> str:
    return shape.draw()

render(Circle())  # "O"
render(Square())  # "[]"

Mypy Static Checking

# Install: pip install mypy
# Run: mypy your_script.py

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

add("hello", "world")  # mypy error: Argument 1 has type "str"

Key Takeaways

1. Add type hints for better IDE support and documentation
2. Use Optional[X] instead of Union[X, None]
3. Use TypeVar for generic functions and classes
4. Use Protocol for structural subtyping
5. Run mypy to catch type errors statically