Add Configure Other Attributes Of Dataclass Field as a Python TIL

python/configure-other-attributes-of-dataclass-field.md

@@ -0,0 +1,63 @@
+# Configure Other Attributes Of Dataclass Field
+
+I have a basic [`dataclass`](https://docs.python.org/3/library/dataclasses.html)
+defined with a few fields that looks like this:
+
+```python
+from dataclasses import dataclass
+from datetime import datetime
+
+@dataclass
+class Session:
+    start_time: datetime
+    project_name: str
+    tags: list[str]
+    end_time: datetime | None = None
+
+    # ...
+
+new_session = Session(start_time, "my-project", [])
+```
+
+This syntax lets me define the fields that make up arguments to the underlying
+`__init__` function. This only gets me so far though. What if I want `tags` to
+have a default value of `[]`? What if I want to force those optional fields
+(`tags` and `end_time`) to be keyword-only arguments?
+
+To take the field definitions of my `dataclass` a bit further, I need the
+[`field` function](https://docs.python.org/3/library/dataclasses.html#dataclasses.field)
+which lets me configure more aspects of each field.
+
+```python
+from dataclasses import dataclass, field
+from datetime import datetime
+
+@dataclass
+class Session:
+    start_time: datetime
+    project_name: str
+    tags: list[str] = field(default_factory=list, kw_only=True)
+    end_time: datetime | None = field(default=None, kw_only=True)
+
+    # ...
+
+sesh1 = Session(start1, "my-project", tags=["pytorch", "numpy"])
+sesh2 = Session(start2, "other-project", end_time=datetime.now())
+```
+
+If I were to say the default value of `tags` is simply a `[]`, then the
+`dataclass` definition gets evaluated with a reference to that specific instance
+of an empty list. Every new `Session` I create would share and mutate that same
+list. For this reason, `dataclass` detects and warns you away from that.
+
+Instead, I specify `list` as the `default_factory` for `tags`. Each new
+`Session` where `tags` is not provided will be factory'd a new empty list.
+
+For scalar values, like `None`, I can use the `default` parameter instead like
+I've done for `end_time`.
+
+As for marking certain fields as keyword-only, the `kw_only` parameter can be
+overridden with `True`.
+
+See [the docs for `field`](https://docs.python.org/3/library/dataclasses.html#dataclasses.field)
+for all the other configurable options.