A poor man's dataclass not using @DataClass for backwards compat with Python 3.6, based on namedtuples.

dataclass_defaults_p36.py

from collections import namedtuple

_Activity = namedtuple('Activity', ['activityId', 'studentId', 'timeRead'])
_Activity.__new__.__defaults__ = (None, None, 0.0)


class Activity(_Activity):
    @staticmethod
    def from_dict(args):
        args = {k: v for k, v in args.items() if k in _Activity._fields}
        return Activity(**args)

    def as_seconds(self, attr) -> float:
        return self.__getattribute__(attr) / 1000 / 60


act = {
    'xactivityId': 'ACT001',
    'xstudentId': 'STU001',
    'createdAt': 1234566,
}

item = Activity.from_dict(act)
print(item)
item = item._replace(timeRead=3105111)
print('---')
print(item)
print(item.as_seconds('timeRead'))