Allow to register new class/dataclass/attribute makers in attrs plugin

[hynek]

This is a feature request.

attrs is very much intended to be composable (e.g. https://github.com/hynek/environ_config) but currently the attrs plugin only works on the core library's functions:

mypy/mypy/plugins/attrs.py

Lines 24 to 37 in b2d6174

	# The names of the different functions that create classes or arguments.
	attr_class_makers = {
	'attr.s',
	'attr.attrs',
	'attr.attributes',
	}
	attr_dataclass_makers = {
	'attr.dataclass',
	}
	attr_attrib_makers = {
	'attr.ib',
	'attr.attrib',
	'attr.attr',
	}

I would like to be able to signal somehow to mypy, that some function is a wrapper around @attr.s/@attr.ib/@attr.dataclass. Not only would third-party applications benefit, but I could really use it while experimenting on python-attrs/attrs#408.

Any ideas/plans how we could achieve that?

cc also @euresti & @chadrik

[euresti]

FYI in our code I literally append to these lists in a mypy plugin.

I did have a vision that the plugin could support an @class_maker(...) decorator that would do nothing at runtime but let the plugin know that this method (along with some default values) is a class_maker or attrib_maker.

So you'd do:

@class_maker(auto_attribs_default=True)
def my_class_maker(...):
   ...

Or something like that.

[ilevkivskyi]

Another possible pattern we can support is:

if TYPE_CHECKING:
    custom_maker = attr.s
else:
    def custom_maker(cls):
        ...

[ilevkivskyi]

The same problem exists for dataclasses, see #6239

[ilevkivskyi]

Just FTR cross-posting the workaround that currently works:

from typing import TYPE_CHECKING
if TYPE_CHECKING:
    from attr import s as custom_maker
else:
    def custom_maker(cls):
        ...

but it is arguably worse than #5406 (comment), that currently doesn't work.

[euresti]

Heh. Here's what I did in our code base. I added a plugin that adds some methods when imported. (Did y'all wonder why I made those variables globals?)

from mypy.plugin import Plugin
from mypy.plugins.attrs import (
    attr_attrib_makers,
    attr_class_makers,
    attr_dataclass_makers,
)

# These work just like attr.dataclass
attr_dataclass_makers.add("my_module.method_looks_like_attr_dataclass")

# This work just like attr.s
attr_class_makers.add("my_module.method_looks_like_attr_s")

# These are our attr.ib makers.
attr_attrib_makers.add("my_module.method_looks_like_attrib")

class MyPlugin(Plugin):
    # Our plugin does nothing but it has to exist so this file gets loaded.
    pass


def plugin(version):
    return MyPlugin