Support factory= in attr plugin.

It works pretty much like default=

mypy/plugins/attrs.py

@@ -318,6 +318,12 @@ def _attribute_from_attrib_maker(ctx: 'mypy.plugin.ClassDefContext',
  init = _get_bool_argument(ctx, rvalue, 'init', True)
  # TODO: Check for attr.NOTHING
  attr_has_default = bool(_get_argument(rvalue, 'default'))
+ attr_has_factory = bool(_get_argument(rvalue, 'factory'))
+
+ if attr_has_default and attr_has_factory:
+ ctx.api.fail("Can't pass both `default` and `factory`.", rvalue)
+ elif attr_has_factory:
+ attr_has_default = attr_has_factory
 
  # If the type isn't set through annotation but is passed through `type=` use that.
  type_arg = _get_argument(rvalue, 'type')

test-data/unit/check-attr.test

@@ -771,3 +771,15 @@ class FFrozen(F):
  def bar(self) -> bool:
  return self._cb(5, 6)
 [builtins fixtures/callable.pyi]
+
+[case testAttrsWithFactory]
+from typing import List
+import attr
+def my_factory() -> int:
+ return 7
+@attr.s
+class A:
+ x: List[int] = attr.ib(factory=list)
+ y: int = attr.ib(factory=my_factory)
+A()
+[builtins fixtures/list.pyi]

test-data/unit/lib-stub/attr.pyi

@@ -13,7 +13,9 @@ def attr(default: Optional[_T] = ...,
  convert: Optional[Callable[[Any], _T]] = ...,
  metadata: Any = ...,
  type: Optional[Type[_T]] = ...,
- converter: Optional[Callable[[Any], _T]] = ...) -> _T: ...
+ converter: Optional[Callable[[Any], _T]] = ...,
+ factory: Optional[Callable[[], _T]] = ...,
+ ) -> _T: ...
 @overload
 def attr(default: None = ...,
  validator: None = ...,
@@ -24,7 +26,9 @@ def attr(default: None = ...,
  convert: Optional[Callable[[Any], _T]] = ...,
  metadata: Any = ...,
  type: None = ...,
- converter: None = ...) -> Any: ...
+ converter: None = ...,
+ factory: Optional[Callable[[], _T]] = ...,
+ ) -> Any: ...
 
 @overload
 def attributes(maybe_cls: _C,