Use keyword-only arguments in Python dataclasses

> that would make using your APIs very annoying

For Objective C, using named parameters is the only way to call methods. I don't think I read many critique about this particular aspect. IMO it's actually a good thing and increases readability quite a bit.

For JavaScript/TypeScript React codebase, using objects as a poor man's named parameters also very popular approach.

Also I'd like to add, that it seems a recent trend to add feature to IDEs, where it'll add hint for every parameter, somewhat simulating named parameters. So when you write `mymethod(value)`, it'll display it as `mymethod(param:value)`.

So may be not very annoying.

The only little thing I'd like to borrow from JavaScript is using "shortcut", so you could replace `x=x` with `x`, if your local variable happened to have the same name, as parameter name (which happens often enough).

> You could argue the same thing (forcing kwargs) for all Python functions/methods, although, that would make using your APIs very annoying. The `__init__` method for dataclasses are just another method like any other.

While that is self evident at a technical level, it is not quite so from a clarity / documentary perspective: “normal” functions and methods can often hint at their parameters through their naming but it is uncommon for types, for which the composite tends to be much more of an implementation detail.

Of course neither rule is universal e.g. the composite is of prime importance for newtypes, and indeed they often use tuple-style types or have special support with no member names.

That's always the challenge when iterating on interfaces that other people depend on.

What we do is go through a deprecation phase. Our process is:

* We provide compatibility with the old signature for 2 major releases.

* We document the change and the timeline clearly in the docstring.

* The function gets decorated with a helper that checks the call, and if any keyword-only arguments are provided as positional, it warns and converts them to keyword-only.

* After 2 major releases, we move fully to the new signature.

We buit a Python library called housekeeping (https://github.com/beanbaginc/housekeeping) to help with this. One of the things it contains is a decorator called `@deprecate_non_keyword_only_args`, which takes a deprecation warning class and a function using the signature we're moving to. That decorator handles the check logic and generates a suitable, consistent deprecation message.

That normally looks like:

    @deprecate_non_keyword_only_args(MyDeprecationWarning)
    def my_func(*, a, b, c):
        ...

But this is a bit more tricky with dataclasses, since `__init__()` is generated automatically. Fortunately, it can be patched after the fact. A bit less clean, but doable.

So here's how we'd handle this case with dataclasses:

    from dataclasses import dataclass
    
    from housekeeping import BaseRemovedInWarning, deprecate_non_keyword_only_args
    
    
    class RemovedInMyProject20Warning(BaseRemovedInWarning):
        product = 'MyProject'
        version = '2.0'
    
    
    @dataclass(kw_only=True)
    class MyDataclass:
        a: int
        b: int
        c: str
    
    MyDataclass.__init__ = deprecate_non_keyword_only_args(
        RemovedInMyProject20Warning
    )(MyDataclass.__init__)

Call it with some positional arguments:

    dc = MyDataclass(1, 2, c='hi')

and you'd get:

    testdataclass.py:26: RemovedInMyProject20Warning: Positional arguments `a`, `b` must be passed as keyword arguments when calling `__main__.MyDataclass.__init__()`. Passing as positional arguments will be required in MyProject 2.0.
      dc = MyDataclass(1, 2, c='hi')

We'll probably add explicit dataclass support to this soon, since we're starting to move to kw_only=True for dataclasses.