[FIXED] Sorting a list of objects by attribute

Issue

I am trying to sort a list of objects in python, however this code will not work:

import datetime

class Day:
    def __init__(self, date, text):
        self.date = date
        self.text = text

    def __cmp__(self, other):
        return cmp(self.date, other.date)

mylist = [Day(datetime.date(2009, 01, 02), "Jan 2"), Day(datetime.date(2009, 01, 01), "Jan 1")]
print mylist
print mylist.sort()

The output of this is:

[<__main__.Day instance at 0x519e0>, <__main__.Day instance at 0x51a08>]
None

Could somebody show me a good way solve this? Why is the sort() function returning None?

Solution

mylist.sort() returns nothing, it sorts the list in place. Change it to

mylist.sort()
print mylist

to see the correct result.

See http://docs.python.org/library/stdtypes.html#mutable-sequence-types note 7.

The sort() and reverse() methods modify the list in place for economy of space when sorting or reversing a large list. To remind you that they operate by side effect, they don’t return the sorted or reversed list.

Answered By - Teemu Kurppa