Use keyword-only arguments in Python dataclasses

28 points by ubernostrum

munyari

Another great dataclass parameter is frozen=True, which marks your dataclasses as immutable. Hopefully the benefits of immutability speak for themselves, but it also makes your dataclass hashable for free, allowing you to use them in sets/as keys dict keys.

Halkcyon

I used to be annoyed by keyword-only in Pydantic and debated changing the default for a long time, but I’ve come around on wanting keyword-only on any class construction because it’s explicit and easy to change which is more valuable to me now than making the code shorter.

Aks

This is such a good tip, thanks for sharing. Some of my projects could have used this. I admit I still don’t know much of the @annotation system in Python.

Is there something else like this, especially for creating enum classes?

Halkcyon

I admit I still don’t know much of the @annotation system in Python.

These are called decorators in Python. They are just callables that take callables as arguments to wrap it, a way to do metaprogramming.
ubernostrum
I admit I still don’t know much of the @annotation system in Python.

This:
```
@some_decorator
def my_function():
    print("Hello")
```
is exactly equivalent to this:
```
def my_function():
    print("Hello")

my_function = some_decorator(my_function)
```
The @ form is just syntactic sugar. You can do it to a function, a method, or a class. There are a lot of use cases, ranging from simple things like adding pre/post-call actions (like logging) to complex transformations, to adding objects to a registry, to… well, anything you can think of that’s legal to do in a Python function.
masklinn

Something like what? https://docs.python.org/3/library/enum.html has been there a while if you need C-ish enums (and / or flags).

If you want something closer to sum types, https://docs.python.org/3/library/typing.html#typing.Union and simple newtypes (e.g. trivial dataclasses with a single member) is probably what you’re looking for.
- Aks
  
  Yeah that was it, thanks! :)
  - KevinMGranger
    
    Don’t get confused though, typing.NewType is type checking -time only, hence their recommendation to use dataclasses.