make-dataclass-sortable-by-specific-field.md

Make Dataclass Sortable By Specific Field

One way to sort a list of some dataclass is to define the key parameter when calling sort or sorted like I discussed in Sort a List of Dataclass Instances:

for date in sessions_grouped_by_day.keys():
    sessions_grouped_by_day[date].sort(
        key=lambda session: session.start_time.time()
    )

But then that lambda for key needs to be defined everywhere you sort.

If the dataclass has a single, specific field that acts as a natural proxy for sort order, then you can define that in the dataclass implementation with the __lt__ method.

As long as a class defines the less than dunder method, it will be sortable.

Here is what that looks like for this Session dataclass:

from dataclasses import dataclass
from datetime import datetime, timezone

@dataclass
class Session:
    start_time: datetime
    project_name: str
    end_time: datetime | None = None

    def __lt__(self, other):
        if not isinstance(other, Session):
            return NotImplemented
        return self.start_time < other.start_time

    # more methods below ...

This implementation of __lt__ tells the sorting methods that this (self) instance of Session can be compared to some other instance of Session by comparing their start_time values to see which is less than. The guard at the beginning makes sure only instances of Session are being compared.