Add Sort A List Of Dataclass Instances as a Python TIL

README.md

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 - [Parse Relative Time To datetime Object](python/parse-relative-time-to-datetime-object.md)
 - [Skip Specific Pytest Test Cases](python/skip-specific-pytest-test-cases.md)
+- [Sort A List Of Dataclass Instances](python/sort-a-list-of-dataclass-instances.md)
 - [Start The Debugger When A Test Errors](python/start-the-debugger-when-a-test-errors.md)
 - [Store And Access Immutable Data In A Tuple](python/store-and-access-immutable-data-in-a-tuple.md)
 - [Test A Function With Pytest](python/test-a-function-with-pytest.md)

python/sort-a-list-of-dataclass-instances.md

@@ -0,0 +1,52 @@
+# Sort A List Of Dataclass Instances
+
+Sorting lists of scalar values (integers, strings, floats, even booleans) in
+Python is simple because the natural ordering of the list elements will be used.
+We can call `sorted` on the list and it _just works_.
+
+```python
+>>> items = ["orange", "apple", "banana", "mango"]
+>>> sorted(items)
+['apple', 'banana', 'mango', 'orange']
+```
+
+However, if we have a list of non-scalar values, it is a little more complex. We
+have to give `sorted` some help with knowing how to sort things that don't have
+a natural ordering.
+
+Let's take this `dataclass` that represents a time-based `Session` as an
+example.
+
+```python
+from dataclasses import dataclass
+from datetime import datetime, timezone
+
+@dataclass
+class Session:
+    start_time: datetime
+    project_name: str
+    end_time: datetime | None = None
+
+    # plus several methods ...
+```
+
+If I have a list of `Session` instances that I want to sort, I have to give
+`sorted` a `key` to sort on. In the case of these `Session` instances, we'll
+pass a `lambda` that can be evaluated to determine the sort value (which needs
+to be sortable). `datetime` instances are sortable and I want to sort these
+sessions based on their `start_time` values.
+
+Here is a snippet from my `py_vmt` CLI where I make sure that each list of
+sessions in this day-by-day `dict` is sorted based on the `start_time`:
+
+```python
+for date in sessions_grouped_by_day.keys():
+    sessions_grouped_by_day[date].sort(
+        key=lambda session: session.start_time.time()
+    )
+```
+
+`sort` (and `sorted`) translates each item in the list to the values produced
+by the lambda and then sorts them by those values.
+
+[source](https://docs.python.org/3/howto/sorting.html)