How to Sort Python Objects

Question

I have a nested list which contains different objects, they're duplicate pairs of objects in the nested list and i'm trying to remove them but i keep getting a

TypeError: unorderable types: practice() < practice()

I know this error is caused by me trying to work with objects rather than integers but i don't know how else to remove the duplicates here is what i tried

class practice:
    id = None

    def __init__(self,id):
        self.id = id

a = practice('a')
b = practice('b')
c = practice('c')
d = practice('d')
e = practice('e')
f = practice('f')

x = [[a,b],[c,d],[a,b],[e,f],[a,b]]

unique_list = list()
for item in x:
    if sorted(item) not in unique_list:
        unique_list.append(sorted(item))

print(unique_list)

Pass in a comparator key to sorted should work.

Luca
– Luca

2015-04-23 00:45:38 +00:00
Commented Apr 23, 2015 at 0:45 — Luca
– Luca, Commented Apr 23, 2015 at 0:45

Padraic Cunningham · Accepted Answer · 2015-04-23 01:10:10Z

6

If you want to compare the objects by the id:

class practice:
    id = None

    def __init__(self,id):
        self.id = id

    def __lt__(self, other):
        return other.id > self.id

    def __gt__(self, other):
        return self.id > other.id

unique_list = list()
for item in x:
    if sorted(item) not in unique_list:
        unique_list.append(sorted(item))

print(unique_list)
[[<__main__.practice object at 0x7fe87e717c88>, <__main__.practice object at 0x7fe87e717cc0>],
 [<__main__.practice object at 0x7fe86f5f79e8>, <__main__.practice object at 0x7fe86f589278>],
 [<__main__.practice object at 0x7fe86f589be0>, <__main__.practice object at 0x7fe86f589c18>]]

Depending on the functionality you want to implement all the rich comparison ordering methods you can use functools.total_ordering, you just need to define one of the methods and it will take care of the rest

from functools import total_ordering
@total_ordering
class practice:
    id = None

    def __init__(self,id):
        self.id = id

    def __lt__(self, other):
        return other.id > self.id

    def __eq__(self, other):
        return self.id == other.id

Given a class defining one or more rich comparison ordering methods, this class decorator supplies the rest. This simplifies the effort involved in specifying all of the possible rich comparison operations:

The class must define one of __lt__(), __le__(), __gt__(), or __ge__(). In addition, the class should supply an __eq__() method.

edited Apr 23, 2015 at 1:10

answered Apr 23, 2015 at 0:47

Padraic Cunningham

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1 Comment

Padraic Cunningham Over a year ago

No prob, I just added the second example as it may help if you want to add more functionality later

Shashank · Accepted Answer · 2015-04-23 00:53:30Z

3

To support sorting without explicit keys for objects in Python 3, you must implement the __lt__ special method:

class practice:
    id = None

    def __init__(self,id):
        self.id = id

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

If you want the other operators to work, you'll have to implement their special methods as well, but for sorting __lt__ is all you need.

As noted in the comments, the other way to do it is to provide an explicit key function to the sorted built-in:

sorted(item, key=lambda x: x.id)

answered Apr 23, 2015 at 0:53

Shashank

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2 Comments

danidee Over a year ago

was trying to use the lambda method, i created a new variable and equated it to sorted(item, key=lambda x: x.id) but when i try to print out the value, i get practice object is not iterable. isn't it supposed to be a list?

Shashank Over a year ago

@danidee The first argument to sorted has to be an iterable of practice objects. So try sorted([practice('b'), practice('a')], key=lambda x: x.id) for example. It will return a new list in the form: [practice('a'), practice('b')]

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