mirror of
https://github.com/jbranchaud/til
synced 2026-08-31 16:51:46 +00:00
48 lines
1.7 KiB
Markdown
48 lines
1.7 KiB
Markdown
# 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.
|