starfield

starfield is a Python package that allows you to create attrs classes with a single field that can be initialized using variadic positional arguments (i.e. the star *). This makes it easier to initialise list-like structures with attrs without having to explicitly pass a list to the initializer.

Installation

To install starfield, run the following command in your terminal:

pip install starfield

Examples

The following example shows how to use starfield to create a class that behaves like a list with some extra fields:

from attrs import define, field
from starfield import starfield


@define(field_transformer=starfield)
class ShoppingList:
    items: list = field(init="*")
    store: str = field()


grocery_list = ShoppingList("Milk", "Bread", "Eggs", store="Supermarket")

Without starfield, you would have to explicitly pass a list to the initializer:

from attrs import define, field


@define
class ShoppingList:
    items: list = field()
    store: str = field()


grocery_list = ShoppingList(["Milk", "Bread", "Eggs"], store="Supermarket")

To illustrate the power of starfield, let's look at a more complex example involving nested fields. Suppose we want to create a data structure to represent a simple grammatical expression:

"I" ( "love" | "hate" ) ( "cats" | "dogs" )

We can define a class to represent this expression with attrs and starfield:

from attrs import define, field
from starfield import starfield


@define(field_transformer=starfield)
class And:
    children: list = field(init="*")


@define(field_transformer=starfield)
class Or:
    children: list = field(init="*")


expr = And("I", Or("love", "hate"), Or("cats", "dogs"))

Without starfield, you would have to explicitly pass a list to the initializer:

from attrs import define, field


@define
class And:
    children: list = field()


@define
class Or:
    children: list = field()


expr = And(["I", Or(["love", "hate"]), Or(["cats", "dogs"])])

Why Use starfield?

Nested fields can quickly become unwieldy when initializing objects with attrs. attrs's documentation explains why it's usually better to use a classmethod than to modify the initializer. But this can make initialization even more verbose.

starfield provides an alternative to using a classmethod by allowing you to define a single field that can be initialized using variadic positional arguments (i.e. the star *).

Features

• starfield will make all non-star fields keyword-only.

• You can still set the star field using a keyword argument (e.g. expr = And("I", items=[Or("love", "hate"), Or("cats", "dogs")])).

• To make the string representation of the class more readable, starfield adds a __rich_repr__ method to the class. However, this only works if you're using rich to stringify your objects. To add a __repr__ method as well, you can pass repr=True to starfield.

Limitations and Caveats

• starfield only works with classes that use attrs.

• You can only have one star field per class.

• The behaviour of starfield's __repr__ method may be inconsistent with the attrs-generated __repr__ methods.

• starfield is not yet mypy-compatible.

  • In particular, mypy will complain about the following:
    • [arg-type]: static type checkers won't recognize variadic positional arguments as valid arguments to the initializer.
    • [call-overload] and [misc] on field(init="*"): attrs expects init to be a bool.
  • As a workaround, you can suppress these errors using:
    • # type: ignore[arg-type] upon initialization with variadic positional arguments, and
    • # type: ignore[call-overload, misc] after field(init="*") in the class definition.

Related Projects

• This feature has been requested and discussed here. The use of init="*" is also proposed.

• pydantic's root types serve a similar purpose. Notable, however, a class with a root type cannot have any other fields.

Please let me know if I've missed any.

Dependencies

starfield requires Python 3.9 or later and attrs >= 22.2.0.

Authors

• Isaac Breen