An attempt to validate data as you read it (something in the mood of PEP 463)

default.py

from typing import TypeVar, Callable, Any, Optional, Tuple, Type

DATA_ERROR = (
    KeyError,
    TypeError,
    ValueError,
    AttributeError,
    IndexError,
    AssertionError,
)


D = TypeVar('D')


def d(
    cb: Callable[[], D],
    default: Optional[D] = None,
    check_type: bool=True,
    exception: Tuple[Type[Exception], ...] = DATA_ERROR,
) -> D:
    try:
        out = cb()

        if check_type and default is not None:
            assert isinstance(out, default.__class__)

        return out
    except exception:
        return default
    except AssertionError:
        return default

example.py

data = {
    'foo': [1, 2, 3],
    'bar': 'fukyou',
}

print(d(lambda: data['foo'], []))
# [1, 2, 3]

print(d(lambda: data['bar'], []))
# []