Python：如何扩展datetime.timedelta

Question

我正在尝试扩展Python，datetime.timedelta以用于越野比赛的结果。我想从格式为string的对象构造一个对象u"mm:ss.s"。我可以使用工厂设计模式和@classmethod注释来完成此操作。我如何通过覆盖__init__和/或完成相同的工作__new__？

使用下面的代码，构造一个对象会引发TypeError。请注意，__init__未调用，因为'in my __init__'未打印。

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"')

这是错误：

>>> 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
>>> 

如果将代码从__init__移至__new__，则会得到以下结果。注意这次，输出显示我的__new__函数被调用。

>>> 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的__new__()方法中设置的-因此您需要覆盖该方法而不是其方法__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：

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')

顺便说一句，您为timestr关键字参数提供默认值会被视为非法并引发，我觉得很奇怪ValueError。