Python class inheritance

Question

Happy new years guys!

I'm new to Python and have been experimenting with class inheritance. I created the code below and have a few questions -

1. Why is shDate3 of type numpy.datetime64 instead of SHDate3? shDate seems to be of type SHDate, which is the behavior I was expecting.
2. Why can't shDate2 be created? I'm receiving "'an integer is required'" error...

Thanks a lot!

from datetime import *
from numpy import *

class SHDate(date):
    def __init__(self, year, month, day):
        date.__init__(self, year, month, day)

class SHDate2(date):
    def __init__(self, dateString):
        timeStruct = strptime(dateString, "%Y-%m-%d")
        date.__init__(self, timeStruct.tm_year, timeStruct.tm_mon, timeStruct.tm_mday)

class SHDate3(datetime64):
    def __init__(self, dateString):
        super(SHDate3, self).__init__(dateString)

if __name__ == '__main__':
    shDate = SHDate(2010,1,31)
    print type(shDate)

    shDate3 = SHDate3("2011-10-11")
    print shDate3
    print type(shDate3)

    shDate2 = SHDate2("2011-10-11")
    print shDate2

Answer 1

Quick answers:

1. Make sure you know when you should use either type or isinstance, they are different. You may want to take a look at this question, it clarifies type and isinstance usage.
2. You shouldn't be using __init__ to custom your date class, because it is an immutable class. This question provides some discussion on customizing instances for those classes.