Make evolve 1st arg positional-only

[ikonst]

It's arbitrary that the 1st argument is named inst and could conflict with a field.

Since PEP-570 positional-only arguments require Python 3.8 and next major version will be ≥ Python 3.7, we can do it the old-fashion way in the next breaking version (as this is an API change), i.e.

def evolve(*args, **changes):
  (inst,) = args
  ...

[hynek]

Wow that's gross :D – I wonder if that's possible to express for typing? It would probably break some people who passed it as keyword arguments… 🤔

[ikonst]

attrs has type stubs so the way it looks to typing can be modern w/PEP-570.

Also, recently I've merged python/mypy#14526 where you can provide the instance only positionally, so in attrs it will be positional only from the next release due to plugin code.

[ikonst]

Dataclasses did something similar, albeit with warning and not a breaking change. Maybe we want to do the same: https://bugs.python.org/issue37163

[hynek]

FWIW, they did break later as announced in the issue:

In [2]: dataclasses.replace?
Signature: dataclasses.replace(obj, /, **changes)
Docstring:
Return a new object replacing specified fields with new values.

This is especially useful for frozen classes.  Example usage::

  @dataclass(frozen=True)
  class C:
      x: int
      y: int

  c = C(1, 2)
  c1 = replace(c, x=3)
  assert c1.x == 3 and c1.y == 2
File:      /Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/dataclasses.py
Type:      function

guess we could warn now and break in a year.