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