Python 3 datetime.date subclass

quarter.py

import copy
import datetime
import pickle

# Each quarter corresponds to the following month and day combinations:
_q1 = (3, 31)
_q2 = (6, 30)
_q3 = (9, 30)
_q4 = (12, 31)

_qs = [_q1, _q2, _q3, _q4]

class Quarter(datetime.date):
    def __new__(cls, year, q):
        if q < 1 or q > 4:
            raise ValueError("Quarter must be 1, 2, 3 or 4")
        month, day = _qs[q-1]
        return super().__new__(cls, year, month, day)

    @property
    def q(self):
        return self.month // 3 # We want an int type

    def __repr__(self):
        return '{0}({1}, {2})'.format(self.__class__.__name__, self.year, self.q)

    def __reduce__(self):
        return (self.__class__, (self.year, self.q))

    # Some example magic methods
    def __str__(self):
        return "{year} Q{q}".format(year=self.year, q=self.q)

    def __add__(self, other):
        """Add an integer number of quarters to the existing quarter."""
        assert isinstance(other, int)
        value = self.year * 4 + self.q + other - 1
        return Quarter(value // 4, value % 4 + 1)


if __name__ == '__main__':
    q1 = Quarter(2018, 1)

    assert eval(repr(q1)) == q1

    # Copy
    assert isinstance(copy.copy(q1), Quarter)
    assert copy.copy(q1) == q1

    # Pickle
    assert isinstance(pickle.loads(pickle.dumps(q1)), Quarter)
    assert pickle.loads(pickle.dumps(q1)) == q1

    # Hashing
    quarters = {q1: True}
    assert quarters[q1]

    # Other methods
    assert q1 < Quarter(2018, 2)
    assert q1 <= Quarter(2018, 1)
    assert q1 > Quarter(2017, 4)
    assert str(q1) == "2018 Q1"
    assert (q1 + 3) == Quarter(2018, 4)
    assert (q1 + 4) == Quarter(2019, 1)