Sort with operator overloading in python

Question

I am new to operator overloading so please bear with me. I have a class with operator overloading __lt__ which is supposed to prints out the sorted result of a list but I'm getting error '<' not supported between instances of 'Inventory' and 'Inventory'. Why can't the sort method sort the contents of the list. Where did I go wrong? Any help is appreciated. Thanks in advance.

class Inventory:
    def __init__(self, item, cost):
        self.item = item
        self.cost = cost
        
    def __str__(self):
        return "{:16s} ${:6.2f}".format(self.item, self.cost)

    def __lt__(self, other):
        return Inventory(self.cost < other.cost)

i1 = Inventory("Pentel Pen", 1.45)
i2 = Inventory("Ruler", .99)
i3 = Inventory("Calculus Text", 245.99)
i4 = Inventory("Diet Coke", 1.80)
t = [i1, i2, i3, i4]
t = [i1, i2, i3, i4]

for i in sorted(t):
    print(i)

Expected result:

Ruler            $  0.99
Pentel Pen       $  1.45
Diet Coke        $  1.80
Calculus Text    $245.99

return Adder(self.value + other.value + random.randrange(1, 6)) — Olvin Roght
– Olvin Roght, Commented May 9, 2021 at 19:38
I accidentally published the wrong question earlier. I have updated my question to the right one. Apologies for the confusion. @OlvinRoght — hamtonko
– hamtonko, Commented May 9, 2021 at 20:11
The code you posted says: "TypeError: __init__() missing 1 required positional argument: 'cost'" because Inventory(self.cost < other.cost) attempts to initialise an instance of Inventory providing self.cost < other.cost as item and nothing as cost. — ForceBru
– ForceBru, Commented May 9, 2021 at 20:14
Do i have to initialize item too something likeInventory(self.item + "," + str(self.cost) < str(other.cost))? @ForceBru — hamtonko
– hamtonko, Commented May 9, 2021 at 20:20
@hamtonko, why? __lt__(self, other) should answer the question: "Is self less than other? Yes or no?" So it should return a Boolean (True or False). There could be reasons to return something else, but I don't think it makes sense here — ForceBru
– ForceBru, Commented May 9, 2021 at 20:23

Jasmijn · Accepted Answer · 2021-05-09 20:55:51Z

2

Replace your method with:

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

inventory1 < inventory2 should return True or False for sorting to work, not an instance of Inventory.

answered May 9, 2021 at 20:55

Jasmijn

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