Use keyword-only arguments in Python dataclasses

Summary

Python dataclasses are a really nice feature for constructing classes that primarily hold or work with data. They can be a good alternative to using dictionaries, since they allow you to add methods, dynamic properties, and subclasses. They can also be a good alternative to building your own class by hand, since they don’t need a custom __init__() that reassigns attributes and provide methods like __eq__() out of the box. One small tip to keeping dataclasses maintainable is to always construct them with kw_only=True, like so: from dataclasses import dataclass @dataclass(kw_only=True) class MyDataClass: x: int y: str z: bool = True This will construct an __init__() that looks like this: class MyDataClass: def __init__( self, *, x: int, y: str, z: bool = True, ) -> None: self.x = x self.y = y self.z = z Instead of: class MyDataClass: def __init__( self, x: int, y: str, z: bool = True, ) -> None: self.x = x self.y = y self.z = z That * in the argument list means everything that follows must be passed as a keyword argument, instead of a positional argument. There are two reasons you probably want to do this: It allows you to reorder the fields on the dataclass without breaking callers. Positional arguments means a caller can use MyDataClass(1, 'foo', False), and if you remove/reorder any of these arguments, you’ll break those callers unexpectedly. By forcing callers to use MyDataClass(x=1, y='foo', z=False), you remove this risk. It allows subclasses to add required fields. Normally, any field with a default value (like z above) will force any fields following it to also have a default. And that includes all fields defined by subclasses. Using kw_only=True gives subclasses the flexibility to decide for themselves which fields must be provided by the caller and which have a default. These reasons are more important for library authors than anything. We spend a lot of time trying to ensure backwards-compatibility and forwards-extensibility in Review Board, so this is an i...