get_attrs.py

from functools import reduce


class A:
    def __init__(self, obj_list):
        self.obj_list = obj_list


class B:
    def __init__(self, number):
        self.number = number


def _getitem(obj, item):
    try:
        return obj.__dict__.__getitem__(item)
    except AttributeError:
        return obj.__getitem__(item)


def _getattrd(obj, default, *args):
    try:
        return reduce(_getitem, args, obj.__dict__)
    except (KeyError, AttributeError, TypeError):
        return default


list_of_bs = [B(1), B(2), B(3)]
obj_a = A(list_of_bs)

print(_getattrd(obj_a, 'default_value', 'obj_list', 2, 'number'))
# output:
# 3

print(_getattrd(obj_a, 'default_value', 'obj_list', 'no_attr', 4))
# output:
# default_value