What is the purpose of sort_index of a Dataclass in Python?

Question

I am learning about Dataclasses but I am confused on the purpose of sort_index and how it actually works.

I can't seem to find any valuable information on it. The official Python documentation doesn't mention it, which is mind boggling.

Here is an example:

@dataclass(order=True)
class Person:
    sort_index: int = field(init=False, repr=False)
    name: str
    age: int
    weight: int = 190
 
    def __post_init__(self):
        self.sort_index = self.weight

So, what is the purpose of sort_index? What is it used for? When do I use it?

Thanks again for taking the time to answer my question. I am new to Python.

Answer 1

Setting a sort_index attribute (or indeed, any identifier—the name is irrelevant) in the __post_init__ method appears to be the value on which comparisons are performed.

There is an implicit setting of the comparison methods (__lt__, __gt__, etc--read about dunder methods if unfamiliar), using the attributes provided in the __post_init__ method first, and if required, the remaining attributes for resolution.

Class constructor

from dataclasses import dataclass, field

@dataclass(order=True)
class Person:
    sort_index: int = field(init=False)
    age: int
      
    
    def __post_init__(self):
        self.sort_index = self.age

first example—attribute age is equal:

>>> p1 = Person(age=10)
>>> p2 = Person(age=10)
>>> p1 == p2
True

Second example—age is greater:

>>> p1 = Person(age=10)
>>> p2 = Person(age=20)
>>> p2 > p1
True

More complex example:

from dataclasses import dataclass, field

@dataclass(order=True)
class Person:
    foo: int = field(init=False, repr=False)
    bar: int = field(init=False, repr=False)
    name: str
    age: int
    weight: int = 190
    
    
    def __post_init__(self):
        self.foo = self.weight
        self.bar = self.age
        
>>> p1 = Person('p1', 10)
>>> p2 = Person('p1', 11)
>>> p2 > p2
True

Reason
foo (weight) is equal for both instances, so comparison is done on bar (age)

Conclusion
The comparisons can be arbitrarily complex, and identifiers are not important.

I highly recommend this video on dataclasses, by ArjanCodes.
Apart from the video, here's a github link to example dataclass code (from the same video).

Hope this helped—I just learned about dataclasses myself.