starfield 0.1.2

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Lets you define attrs classes with a single non-keyword-only field that can be initialised using var-positional (i.e. star *) arguments.

Installation

pip install starfield

Usage

from attrs import define, field
from starfield import starfield


@define(field_transformer=starfield)
class SantaList:
    names: list = field(init="*")
    is_naughty_list: bool = field()


naughty_list = SantaList("Bob", "Alice", is_naughty_list=True)

Why?

Sometimes you want to define a class that behaves like a list with some extra fields. Without an initializer with var-positional arguments, you would have to explicitly pass a list to the initializer:

from attrs import define, field


@define
class SantaList:
    names: list = field()
    is_naughty_list: bool = field()


naughty_list = SantaList(["Bob", "Alice"], is_naughty_list=True)

This can get messy, especially if you have lots of nested fields.

attrs's documentation recommends explains why it's usually better to use a classmethod than to modify the initializer.

Passing complex objects into init and then using them to derive data for the class unnecessarily couples your new class with the old class which makes it harder to test and also will cause problems later.

Generally speaking, the moment you think that you need finer control over how your class is instantiated than what attrs offers, it’s usually best to use a classmethod factory or to apply the builder pattern.

Nested fields

To motivate starfield more strongly, 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:

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"])])

This works but it's a bit awkward to have to manually pass the list at every level.

Using starfield we can define the same class but with a much simpler initializer:

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"))

How?

The starfield adds to the class an __init__ method that accepts variadic positional arguments. The __init__ method calls __attrs_init__ with the variadic positional arguments passed as a tuple to the field with init="*".

String representation

To make the string representation of the class more readable, starfield also 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. However, its behaviour may be inconsistent with the attrs-generated __repr__ methods which are more complicated than one might expect (and may change without warning - hence why starfield doesn't touch it unless you explicitly ask it to).

Notes

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

• You can still set the star field using a keyword argument (e.g. SantaList(names=["Bob", "Alice"], is_naughty_list=True)).

Similar 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.