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