How to Use Python Type Hints for Beginners

from typing import Any, Union, Optional, List, Dict, Tuple, Callable, TypeVar

T = TypeVar("T")

def describe(value: Any) -> str:
    """Return a human-readable description of the value's type."""
    if isinstance(value, list):
        return f"list of {len(value)} items"
    elif isinstance(value, dict):
        return f"dict with {len(value)} keys"
    elif isinstance(value, str):
        return f"string ({len(value)} chars)"
    elif isinstance(value, (int, float)):
        return f"number: {value}"
    elif value is None:
        return "None"
    else:
        return f"instance of {type(value).__name__}"

def double(x: Union[int, float]) -> Union[int, float]:
    """Double a number."""
    return x * 2

def greet(name: Optional[str] = None) -> str:
    """Greet a person, or everyone if no name given."""
    if name is None:
        return "Hello, everyone!"
    return f"Hello, {name}!"

def process_items(items: List[int]) -> Dict[str, int]:
    """Summarize a list of numbers."""
    return {"total": sum(items), "count": len(items), "max": max(items) if items else 0}

if __name__ == "__main__":
    print(describe([1, 2, 3]))
    print(describe({"a": 1}))
    print(double(5))
    print(greet())
    print(greet("Alice"))
    print(process_items([3, 7, 2]))