如何扩展datetime.timedelta？

Question

我正在尝试扩展Python datetime.timedelta以用于越野比赛结果。我想从u"mm:ss.s"格式的字符串构造一个对象。我可以使用factory设计模式和@classmethod注释来完成这项工作。我如何通过重写__init__和/或__new__来实现同样的目标

使用下面的代码，构造对象会引发TypeError。注意__init__没有被调用，因为'in my __init__'没有被打印

import datetime
import re

class RaceTimedelta(datetime.timedelta):
    def __init__(self, timestr = ''):
        print 'in my __init__'
        m = re.match(r'(\d+):(\d+\.\d+)', timestr)
        if m:
            mins = int(m.group(1))
            secs = float(m.group(2))
            super(RaceTimedelta, self).__init__(minutes = mins, seconds = secs)
        else:
            raise ValueError('timestr not in format u"mm:ss.d"')

以下是错误：

>>> from mytimedelta import RaceTimedelta
>>> RaceTimedelta(u'24:45.7')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported type for timedelta days component: unicode
>>> 

如果我将代码从__init__移动到__new__，我会得到以下结果。注意，这一次，输出显示调用了my__new__函数

>>> RaceTimedelta(u'24:45.7')
in my __new__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "mytimedelta.py", line 16, in __new__
    super(RaceTimedelta, self).__new__(minutes = mins, seconds = secs)
TypeError: datetime.timedelta.__new__(): not enough arguments
>>> 

Answer 1

显然timedelta对象是不可变的，这意味着它们的值实际上是在类__new__()方法中设置的，因此您需要重写该方法而不是它的__init__()：

import datetime
import re

class RaceTimedelta(datetime.timedelta):
    def __new__(cls, timestr=''):
        m = re.match(r'(\d+):(\d+\.\d+)', timestr)
        if m:
            mins, secs = int(m.group(1)), float(m.group(2))
            return super(RaceTimedelta, cls).__new__(cls, minutes=mins, seconds=secs)
        else:
            raise ValueError('timestr argument not in format "mm:ss.d"')

print RaceTimedelta(u'24:45.7')

输出：

0:24:45.700000

顺便说一句，我觉得奇怪的是，您为timestr关键字参数提供了一个默认值，该参数将被视为非法并引发ValueError