Infer empty list without annotation for __slots__ and module __all__ (#19348)

Fixes #10870, fixes #10103.

This adds a fake `Iterable[str]` context when checking the following: 

* `__all__ = []` at top level
* `__slots__ = []` at class level (also works for sets but not for
dicts)

Additionally, this fixes a bug with `__slots__` being mistakenly checked
in other contexts (at top level or in function bodies), so e.g. the
following is now accepted:

```python
def foo() -> None:
    __slots__ = 1
```

Author: sterliakov

mypy/checker.py

@@ -3138,7 +3138,7 @@ def check_assignment(
                         else:
                             self.check_getattr_method(signature, lvalue, name)
 
-                if name == "__slots__":
+                if name == "__slots__" and self.scope.active_class() is not None:
                     typ = lvalue_type or self.expr_checker.accept(rvalue)
                     self.check_slots_definition(typ, lvalue)
                 if name == "__match_args__" and inferred is not None:
@@ -3317,6 +3317,12 @@ def get_variable_type_context(self, inferred: Var, rvalue: Expression) -> Type |
                     type_contexts.append(base_type)
         # Use most derived supertype as type context if available.
         if not type_contexts:
+            if inferred.name == "__slots__" and self.scope.active_class() is not None:
+                str_type = self.named_type("builtins.str")
+                return self.named_generic_type("typing.Iterable", [str_type])
+            if inferred.name == "__all__" and self.scope.is_top_level():
+                str_type = self.named_type("builtins.str")
+                return self.named_generic_type("typing.Sequence", [str_type])
             return None
         candidate = type_contexts[0]
         for other in type_contexts:

mypy/checker_shared.py

@@ -334,6 +334,10 @@ def current_self_type(self) -> Instance | TupleType | None:
                 return fill_typevars(item)
         return None
 
+    def is_top_level(self) -> bool:
+        """Is current scope top-level (no classes or functions)?"""
+        return len(self.stack) == 1
+
     @contextmanager
     def push_function(self, item: FuncItem) -> Iterator[None]:
         self.stack.append(item)

test-data/unit/check-modules.test

@@ -423,7 +423,35 @@ import typing
 __all__ = [1, 2, 3]
 [builtins fixtures/module_all.pyi]
 [out]
-main:2: error: Type of __all__ must be "Sequence[str]", not "list[int]"
+main:2: error: List item 0 has incompatible type "int"; expected "str"
+main:2: error: List item 1 has incompatible type "int"; expected "str"
+main:2: error: List item 2 has incompatible type "int"; expected "str"
+
+[case testAllMustBeSequenceStr2]
+import typing
+__all__ = 1  # E: Type of __all__ must be "Sequence[str]", not "int"
+reveal_type(__all__)  # N: Revealed type is "builtins.int"
+[builtins fixtures/module_all.pyi]
+
+[case testAllMustBeSequenceStr3]
+import typing
+__all__ = set()  # E: Need type annotation for "__all__" (hint: "__all__: set[<type>] = ...") \
+                 # E: Type of __all__ must be "Sequence[str]", not "set[Any]"
+reveal_type(__all__)  # N: Revealed type is "builtins.set[Any]"
+[builtins fixtures/set.pyi]
+
+[case testModuleAllEmptyList]
+__all__ = []
+reveal_type(__all__)  # N: Revealed type is "builtins.list[builtins.str]"
+[builtins fixtures/module_all.pyi]
+
+[case testDunderAllNotGlobal]
+class A:
+    __all__ = 1
+
+def foo() -> None:
+    __all__ = 1
+[builtins fixtures/module_all.pyi]
 
 [case testUnderscoreExportedValuesInImportAll]
 import typing

test-data/unit/check-slots.test

@@ -496,6 +496,29 @@ class A:
         self.missing = 3
 [builtins fixtures/dict.pyi]
 
+[case testSlotsNotInClass]
+# Shouldn't be triggered
+__slots__ = [1, 2]
+reveal_type(__slots__)  # N: Revealed type is "builtins.list[builtins.int]"
+
+def foo() -> None:
+    __slots__ = 1
+    reveal_type(__slots__)  # N: Revealed type is "builtins.int"
+
+[case testSlotsEmptyList]
+class A:
+    __slots__ = []
+    reveal_type(__slots__)  # N: Revealed type is "builtins.list[builtins.str]"
+
+reveal_type(A.__slots__)  # N: Revealed type is "builtins.list[builtins.str]"
+
+[case testSlotsEmptySet]
+class A:
+    __slots__ = set()
+    reveal_type(__slots__)  # N: Revealed type is "builtins.set[builtins.str]"
+
+reveal_type(A.__slots__)  # N: Revealed type is "builtins.set[builtins.str]"
+[builtins fixtures/set.pyi]
 
 [case testSlotsWithAny]
 from typing import Any