Question

For my Python 2.7.3 project I have a class called custom_date, which has a property called fixed_date:

from datetime import date
class custom_date():
    def __init__(self, fixed_date):
        self.fixed_date = fixed_date

    def __lt__(self, other):
        return self.fixed_date < other

    #__gt__, __ge__, __le__, __eq__, __ne__ all implemented the same way

My idea is to be able to directly compare custom_date.fixed_date with the builtin date.

Problem

If I compare a custom_date object to a date object, it’s fine. However, if I compare a date object to a custom_date, it returns a TypeError

>>> from datetime import date
>>> x = custom_date(date(2013,2,1))
>>> y = date(2013,2,2)
>>> x > y
False
>>> y > x
TypeError: can't compare datetime.date to instance 

Is there any way around this?

Answer 1

Just subclass date to get this functionality. Since a datetime object is immutable, you need to use the __new__ constructor vs __init__:

from datetime import date
class custom_date(date):
    def __new__(cls, year,month,day):
        return date.__new__(cls, year, month,day)

x = custom_date(2013,2,1)
y = date(2013,2,2)

print x<y
print y<x

Prints:

True
False

Since the determinative class for comparisons is the LH class, the class on the left needs to have the correct comparison operators to deal with the comparison with the class on the right. If there are no comparison operators for either class, instances are sorted by identity — their memory address. Your error is essentially from trying to compare an apple to an orange: identity to a date class.

Note that there used to be a rcmp that was removed in Python 2.1 to deal with such issues. Introduction of the new style classes and rich comparisons have also led to __cmp__ being deprecated.