Add Another Way To Mark Keyword-Only Dataclass Fields as a Python TIL

python/another-way-to-mark-keyword-only-dataclass-fields.md

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+# Another Way To Mark Keyword-Only Dataclass Fields
+
+In [Configure Other Attributes Of Dataclass
+Field](configure-other-attributes-of-dataclass-field), I showed how the
+[`dataclasses.field`](https://docs.python.org/3/library/dataclasses.html#dataclasses.field)
+constructor function can be used. One of the parameters I demonstrated was
+`kw_only`. Each field constructed with `kw_only=True` will be required to be
+passed as a keyword-only parameter when constructing an instance of that
+`dataclass`.
+
+Another way to specify keyword-only parameters with `dataclass` fields is to
+segment them with `KW_ONLY`. This sentinel value can be included as a
+pseudo-field where all fields that come after it are treated as keyword-only.
+
+Translating the example from that other post would look like this:
+
+```python
+from dataclasses import dataclass, field, KW_ONLY
+from datetime import datetime
+
+@dataclass
+class Session:
+    start_time: datetime
+    project_name: str
+    _: KW_ONLY
+    tags: list[str] = field(default_factory=list, kw_only=True)
+    end_time: datetime | None = None
+
+    # ...
+
+sesh1 = Session(start1, "my-project", tags=["pytorch", "numpy"])
+sesh2 = Session(start2, "other-project", end_time=datetime.now())
+```
+
+The field whose value is `KW_ONLY` is only used to signal that keyword-only
+boundary. It does not itself become a field of the dataclass.
+
+On the one hand I like this approach because it feels closer to [the way this is
+signaled in standard function definition
+syntax](force-remaining-arguments-to-be-named.md).
+
+```python
+def build_session(start_time, project_name, *, tags, end_time=None)
+```
+
+On the other hand, it feels like magic `dataclass` syntax whereas the
+`kw_only=True` parameter is more explicit.