Python 3.8 datetime夏令时转换是否存在Bug？

问题场景 #

尝试将带Europe/Sofia时区信息的datetime转换为该时区当日起始时间（午夜），再转回UTC时出现异常。原测试代码及运行输出如下：

原测试代码 #

#!/usr/bin/env python

import sys
import pytz
from datetime import datetime, timedelta


def main():
    tz = pytz.timezone('Europe/Sofia')
    dt = pytz.utc.localize(datetime(year=2019, month=3, day=31, hour=14, minute=45))
    print(f'We start with the datetime in UTC: {dt}')
    dt = dt.astimezone(tz)
    print(f'Then convert it to Europe/Sofia timezone: {dt}')

    result = dt.replace(hour=0, minute=0, second=0, microsecond=0)
    print(f'Then make it the start of the day midnight: {result}')
    result = result.astimezone(pytz.utc)
    print(f'Then convert it back to UTC (the final result we want): {result}')
    print(f'But notice the time is wrong now when converted back to Europe/Sofia: {result.astimezone(tz)}')

    result = dt - timedelta(hours=dt.hour, minutes=dt.minute, seconds=dt.second, microseconds=dt.microsecond)
    print(f'Using a timedelta instead of replace has the same result: {result}')
    print(f'The time is still wrong when converted back to Europe/Sofia: {result.astimezone(pytz.utc).astimezone(tz)}')

    return 0


if __name__ == '__main__':
    sys.exit(main())

运行输出 #

We start with the datetime in UTC: 2019-03-31 14:45:00+00:00
Then convert it to Europe/Sofia timezone: 2019-03-31 17:45:00+03:00
Then make it the start of the day midnight: 2019-03-31 00:00:00+03:00
Then convert it back to UTC (the final result we want): 2019-03-30 21:00:00+00:00
But notice the time is wrong now when converted back to Europe/Sofia: 2019-03-30 23:00:00+02:00
Using a timedelta instead of replace has the same result: 2019-03-31 00:00:00+03:00
The time is still wrong when converted back to Europe/Sofia: 2019-03-30 23:00:00+02:00

问题核心：2019年3月31日凌晨3点Europe/Sofia时区有夏令时切换（偏移从+2变为+3），预期当日午夜的偏移应为+2，但用replace()或timedelta得到的是+3偏移，导致转回时区后时间错误。

原因分析 #

这不是Python的Bug，是pytz处理带时区datetime的特性：

• 调用replace()或通过timedelta计算时，仅修改了时间字段，没有重新计算时区偏移。原dt的偏移是+3（夏令时切换后的时间），修改后得到的2019-03-31 00:00:00仍沿用+3偏移，但该时间点在该时区实际还未进入夏令时，偏移应为+2，属于无效时间，转回UTC再转回来时会被自动修正为有效时间（2019-03-30 23:00+02）。

解决方案 #

要正确获取夏令时切换当日的午夜时间，需使用pytz的localize()方法，或借助更友好的时区处理库：

方案1：使用pytz.localize()方法 #

提取日期部分后，用时区本地化当日午夜，is_dst=None会强制校验时间有效性：

result = tz.localize(datetime(dt.year, dt.month, dt.day), is_dst=None)

此方法会自动匹配该日期午夜对应的正确时区偏移，得到2019-03-31 00:00:00+02:00。

方案2：使用python-dateutil库（更简洁） #

dateutil.tz会自动处理夏令时切换的偏移问题，无需手动调用localize()：

from dateutil import tz

tz = tz.gettz('Europe/Sofia')
dt = datetime(year=2019, month=3, day=31, hour=14, minute=45, tzinfo=tz.UTC)
dt = dt.astimezone(tz)
result = dt.replace(hour=0, minute=0, second=0, microsecond=0)

修改后的测试代码示例 #

#!/usr/bin/env python

import sys
import pytz
from datetime import datetime, timedelta


def main():
    tz = pytz.timezone('Europe/Sofia')
    dt = pytz.utc.localize(datetime(year=2019, month=3, day=31, hour=14, minute=45))
    print(f'We start with the datetime in UTC: {dt}')
    dt = dt.astimezone(tz)
    print(f'Then convert it to Europe/Sofia timezone: {dt}')

    # 正确方式：用localize构建当日午夜
    result = tz.localize(datetime(dt.year, dt.month, dt.day), is_dst=None)
    print(f'Correct start of the day midnight: {result}')
    result = result.astimezone(pytz.utc)
    print(f'Convert back to UTC: {result}')
    print(f'Convert back to Europe/Sofia (now correct): {result.astimezone(tz)}')

    return 0


if __name__ == '__main__':
    sys.exit(main())

修改后运行输出 #

We start with the datetime in UTC: 2019-03-31 14:45:00+00:00
Then convert it to Europe/Sofia timezone: 2019-03-31 17:45:00+03:00
Correct start of the day midnight: 2019-03-31 00:00:00+02:00
Convert back to UTC: 2019-03-30 22:00:00+00:00
Convert back to Europe/Sofia (now correct): 2019-03-31 00:00:00+02:00

内容的提问来源于stack exchange，提问作者Alan Hamlett