Issue with `attrs` custom `__init__` defined

[George-Ogden]

Bug Report
__init__ definition is ignored when generating attrs.define custom version.

To Reproduce

import attrs


@attrs.define
class A:
    x: int

    def __init__(self, x: str) -> None: ...


@attrs.define(init=False)
class B:
    x: int

    def __init__(self, x: str) -> None: ...

Expected Behavior

A(1) # error: Argument 1 to "A" has incompatible type "int"; expected "str"  [arg-type]
A("1") 
B(1) # error: Argument 1 to "B" has incompatible type "int"; expected "str"  [arg-type]
B("1")

Actual Behavior

A("1") # error: Argument 1 to "A" has incompatible type "str"; expected "int"  [arg-type]
B(1) # error: Argument 1 to "B" has incompatible type "int"; expected "str"  [arg-type]
B("1")

Your Environment

• Mypy version used: mypy 1.20.0+dev
• Mypy command-line flags: file.py --show-traceback
• Mypy configuration options from mypy.ini (and other config files): NA
• Python version used: Python 3.13.9

Notes
I think the issue comes from:

mypy/mypy/plugins/attrs.py

Line 345 in a86f7e3

init = _get_decorator_bool_argument(ctx, "init", True)

as it does not check whether the __init__ method exists, it only checks that init=False when attrs.define is called.

[A5rocks]

Looking at the docs for attrs, init=None (rather than True/False) means it'll detect whether an __init__ is there, based on auto_detect=True (default). The logic here is actually a little bit complicated, quoting the docs:

Instead of setting the init, repr, eq, and hash arguments explicitly, assume they are set to True unless any of the involved methods for one of the arguments is implemented in the current class (meaning, it is not inherited from some base class).

So, for example by implementing __eq__ on a class yourself, attrs will deduce eq=False and will create neither __eq__ nor __ne__ (but Python classes come with a sensible __ne__ by default, so it should be enough to only implement __eq__ in most cases).

Passing True or False to init, repr, eq, or hash overrides whatever auto_detect would determine.