Question

I’m trying to extend Python’s datetime.datetime class with a couple of extra methods. So, for example I’m doing:

import datetime

class DateTime(datetime.datetime):
    def millisecond(self):
        return self.microsecond/1000

but then if I do

>>> d = DateTime(2010, 07, 11, microsecond=3000)
>>> print d.millisecond()
3
>>> delta = datetime.timedelta(hours=4)
>>> newd = d + delta
>>> print newd.millisecond()
AttributeError: 'datetime.datetime' object has no attribute 'millisecond'

This is obviously because doing d + delta calls the datetime.datetime.__add__() method which returns a datetime.datetime object.

Is there any way I can make this datetime.datetime object convert to a DateTime object? Or would I have to reimplement all the operators in my DateTime subclass to return the correct type?

Answer 1

In this case I’d prefer simple free-standing functions:

import datetime
def millisecond(dt):
    return dt.microsecond/1000

Mixins are possible in Python (see the comment), but I think in such cases they are superfluous.