How to Validate Data in Python with Typing Hints

from typing import Any, Optional, Union, TypeVar, get_origin, get_args

T = TypeVar("T")

def validate(value: Any, expected_type: type) -> Optional[str]:
    """Returns an error message if value doesn't match expected_type, else None."""
    # Handle Optional[...] types
    origin = get_origin(expected_type)
    if origin is Union:
        args = get_args(expected_type)
        if type(None) in args and value is None:
            return None
        # Check if value matches any of the Union members
        for arg in args:
            if arg is not type(None) and validate(value, arg) is None:
                return None
        return f"Value {value!r} does not match any of {expected_type}"
    
    # Handle list[...], dict[...], etc.
    if origin is not None:
        if not isinstance(value, origin):
            return f"Expected {origin.__name__}, got {type(value).__name__}"
        # Validate container elements
        args = get_args(expected_type)
        if origin is list and args:
            for item in value:
                error = validate(item, args[0])
                if error:
                    return f"List element error: {error}"
        return None
    
    # Basic type check
    if expected_type is Any:
        return None
    if not isinstance(value, expected_type):
        return f"Expected {expected_type.__name__}, got {type(value).__name__}"
    return None


def validate_dict(data: dict, schema: dict) -> list[str]:
    """Validates a dict against a schema of {field: type}."""
    errors = []
    for field, expected_type in schema.items():
        if field not in data:
            errors.append(f"Missing required field: {field}")
            continue
        error = validate(data[field], expected_type)
        if error:
            errors.append(f"Field '{field}': {error}")
    return errors


if __name__ == "__main__":
    user_schema = {
        "name": str,
        "age": int,
        "email": Optional[str],
        "tags": list[str],
    }
    
    test_users = [
        {"name": "Alice", "age": 30, "email": "alice@example.com", "tags": ["admin"]},
        {"name": "Bob", "age": "thirty", "tags": [1, 2]},
        {"name": "Charlie", "age": 25},
    ]
    
    for user in test_users:
        errors = validate_dict(user, user_schema)
        if errors:
            print(f"User {user.get('name', '?')}: INVALID -> {errors}")
        else:
            print(f"User {user['name']}: VALID")