Add Make Dataclass Sortable By Specific Field as a Python TIL

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

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+# 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](sort-a-list-of-dataclass-instances.md):
+
+```python
+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:
+
+```python
+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.