python - How to extend datetime.timedelta?

Question

I am trying to extend the Python datetime.timedelta for use with cross country race results. I want to construct an object from a string in format u"mm:ss.s". I am able to accomplish this using the factory design pattern and @classmethod annotation. How would I accomplish the same by overriding __init__ and/or __new__?

With the code below, constructing an object raises a TypeError. Note that __init__ is not called, because 'in my __init__' is not printed.

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"')

Here is the error:

>>> 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
>>> 

If I move my code from __init__ to __new__, I get the following. Note that this time, the output shows that my __new__ function is called.

>>> 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

Apparently timedelta objects are immutable, which means their value is actually set in the class' __new__() method—so you'll need to override that method instead of its __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')

Output:

0:24:45.700000

BTW, I find it odd that you're providing a default value for thetimestrkeyword argument that will be considered illegal and raise aValueError.