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Modern Type Annotations

Python is dynamically typed, meaning variable types are verified at runtime. Starting with Python 3.5 (PEP 484), Python introduced optional type hints (type annotations) to document function signatures, enable IDE autocompletion, and allow static analysis tools (such as Mypy) to catch bugs before execution.

Basic Type Annotations

# Variables
name: str = "Alice"
age: int = 30
is_active: bool = True
price: float = 19.99

# Function parameters and return value
def calculate_total(subtotal: float, tax_rate: float) -> float:
    return subtotal * (1 + tax_rate)

Built-In Generic Types (Python 3.9+)

Starting with Python 3.9, built-in collection types (list, dict, set, tuple) can be parameterized directly without importing from the typing module:

Example: Modern Collection Types
def get_user_scores(scores: dict[str, list[int]]) -> dict[str, float]:
    return {user: sum(vals) / len(vals) for user, vals in scores.items()}

sample = {"Alice": [90, 85, 95], "Bob": [70, 75, 80]}
print(get_user_scores(sample))
Output
{'Alice': 90.0, 'Bob': 75.0}

The Union Operator | (Python 3.10+)

Use the pipe operator | to represent multiple acceptable types:

# Accepts either an int or a float
def process_number(x: int | float) -> str:
    return f"Value: {x}"

# Optional type (type or None)
def find_record(record_id: int) -> dict[str, str] | None:
    if record_id == 1:
        return {"name": "Admin"}
    return None

Specific Types: Literal, TypedDict, and Callable

Literal: Specific Allowed Values

from typing import Literal

def set_mode(mode: Literal["read", "write", "append"]):
    print("Mode set to:", mode)

set_mode("read") # Valid

TypedDict: Dictionary with Fixed Key Types

from typing import TypedDict

class UserProfile(TypedDict):
    id: int
    username: str
    is_staff: bool

user: UserProfile = {"id": 1, "username": "admin", "is_staff": True}

Callable: Function Types

from typing import Callable

def apply_func(x: int, func: Callable[[int], int]) -> int:
    return func(x)

print(apply_func(5, lambda n: n * 2))  # 10

Static Type Checking with Mypy

To run static type checking across your project, install and run Mypy:

pip install mypy
mypy script.py