つらい。

datetime.strptime の中身見てみたらものすごい武闘派だったので早々に諦めて、
ざっくり欲しいとこだけ実装することにした。

いっそもう皇紀にしたらいいんじゃないかな。

jp_era.py

import datetime


class JpEra:
    def __init__(self, shorthand, era_name, begin_date, end_date=datetime.date.max):
        self.shorthand = shorthand
        self.era_name = era_name
        self.begin_date = begin_date
        self.end_date = end_date

    def in_term(self, target_date):
        return self.begin_date <= target_date <= self.end_date

    def jp_year(self, year):
        return year - (self.begin_date.year - 1)


JP_ERA = (
    JpEra("M", "明治", datetime.date(1868,  9,  8), datetime.date(1912,  7, 29)),
    JpEra("T", "大正", datetime.date(1912,  7, 30), datetime.date(1926, 12, 24)),
    JpEra("S", "昭和", datetime.date(1926, 12, 25), datetime.date(1989,  1, 7)),
    JpEra("H", "平成", datetime.date(1989,  1,  8)),
)


def parse_jp_datetime(target_str, fmt='%y.%m.%d %H:%M'):
    """
    '元年' がめんどくさいのでとりあえずショートハンドのみ
    :target_str: 'H30.01.15 09:52'
    :fmt:
    :return: datetime.datetime
    """
    x, xs = target_str[0], target_str[1:]
    for era in JP_ERA:
        if x != era.shorthand:
            continue

        # 2100年以降とか元号が３桁になったりするとおかしくなるけどまぁいいや
        c = datetime.datetime.strptime(xs, fmt)
        y = era.begin_date.year + c.year % 100 - 1
        return datetime.datetime(y, c.month, c.day, c.hour, c.minute, c.second)

    raise ValueError('unknown era `%s`.' % x)


def jp_era(target_date):
    """
    :target_date: datetime.date(2018, 1, 15)
    :return: (shorthand, era_name, jp_year)
    """
    for era in JP_ERA:
        if era.in_term(target_date):
            return era.shorthand, era.era_name, era.jp_year(target_date.year)

    raise ValueError('should not be reached here `%s`.' % target_date)

test_jp_era.py

import unittest

import datetime
from jp_era import *


class TestJpEra(unittest.TestCase):
    def test_jp_era(self):
        args_and_expects = [
            [[datetime.date(2018, 1, 15)], ('H', '平成', 30)],
            [[datetime.date(1989, 1, 7)], ('S', '昭和', 64)],
            [[datetime.date(1989, 1, 8)], ('H', '平成', 1)],
        ]

        for args, expect in args_and_expects:
            self.assertEqual(jp_era(*args), expect)

        self.assertRaises(ValueError, lambda: jp_era(
            datetime.date(1000, 1, 1)))

    def test_parse_jp_date(self):
        args_and_expects = [
            [['H30.01.15 09:52'], datetime.datetime(2018, 1, 15, 9, 52)],
            [['H01.01.8 10:00'], datetime.datetime(1989, 1, 8, 10, 00)],
            [['S64.01.7 10:00'], datetime.datetime(1989, 1, 7, 10, 00)],
            [['S90.01.15 09:52'], datetime.datetime(2015, 1, 15, 9, 52)],   # NOTE: 議論の余地がある
        ]

        for args, expect in args_and_expects:
            self.assertEqual(parse_jp_datetime(*args), expect)

        self.assertRaises(
            ValueError, lambda: parse_jp_datetime('X10.01.15 09:52'))


if __name__ == "__main__":
    unittest.main()