Prevent assignment to None for non-Optional class variables (#20054)

Python 3.5 has been dead for a long time. This is just a correct thing
to do.

Author: hauntsaninja

mypy/checker.py

@@ -3283,7 +3283,7 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
         # as X | Y.
         if not (s.is_alias_def and self.is_stub):
             with self.enter_final_context(s.is_final_def):
-                self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
+                self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None)
 
         if s.is_alias_def:
             self.check_type_alias_rvalue(s)
@@ -3331,11 +3331,7 @@ def check_type_alias_rvalue(self, s: AssignmentStmt) -> None:
         self.store_type(s.lvalues[-1], alias_type)
 
     def check_assignment(
-        self,
-        lvalue: Lvalue,
-        rvalue: Expression,
-        infer_lvalue_type: bool = True,
-        new_syntax: bool = False,
+        self, lvalue: Lvalue, rvalue: Expression, infer_lvalue_type: bool = True
     ) -> None:
         """Type check a single assignment: lvalue = rvalue."""
         if isinstance(lvalue, (TupleExpr, ListExpr)):
@@ -3413,15 +3409,6 @@ def check_assignment(
                             # We are replacing partial<None> now, so the variable type
                             # should remain optional.
                             self.set_inferred_type(var, lvalue, make_optional_type(fallback))
-                elif (
-                    is_literal_none(rvalue)
-                    and isinstance(lvalue, NameExpr)
-                    and isinstance(lvalue.node, Var)
-                    and lvalue.node.is_initialized_in_class
-                    and not new_syntax
-                ):
-                    # Allow None's to be assigned to class variables with non-Optional types.
-                    rvalue_type = lvalue_type
                 elif (
                     isinstance(lvalue, MemberExpr) and lvalue.kind is None
                 ):  # Ignore member access to modules
@@ -5303,9 +5290,7 @@ def visit_operator_assignment_stmt(self, s: OperatorAssignmentStmt) -> None:
             # There is no __ifoo__, treat as x = x <foo> y
             expr = OpExpr(s.op, s.lvalue, s.rvalue)
             expr.set_line(s)
-            self.check_assignment(
-                lvalue=s.lvalue, rvalue=expr, infer_lvalue_type=True, new_syntax=False
-            )
+            self.check_assignment(lvalue=s.lvalue, rvalue=expr, infer_lvalue_type=True)
         self.check_final(s)
 
     def visit_assert_stmt(self, s: AssertStmt) -> None:

mypyc/test-data/irbuild-classes.test

@@ -1057,7 +1057,7 @@ class B(A):
     y = LOL
     z: Optional[str] = None
     b = True
-    bogus = None  # type: int
+    bogus = None  # type: int  # type: ignore
 [out]
 def A.lol(self):
     self :: __main__.A

mypyc/test-data/run-classes.test

@@ -1159,7 +1159,7 @@ class B(A):
     y = LOL
     z = None  # type: Optional[str]
     b = True
-    bogus = None  # type: int
+    bogus = None  # type: int  # type: ignore
 
 def g() -> None:
     a = A()

test-data/unit/check-optional.test

@@ -236,7 +236,8 @@ class C:
 [case testMultipleAssignmentNoneClassVariableInInit]
 from typing import Optional
 class C:
-    x, y = None, None  # type: int, str
+    x, y = None, None  # type: int, str  # E: Incompatible types in assignment (expression has type "None", variable has type "int") \
+                                         # E: Incompatible types in assignment (expression has type "None", variable has type "str")
     def __init__(self) -> None:
         self.x = None  # E: Incompatible types in assignment (expression has type "None", variable has type "int")
         self.y = None  # E: Incompatible types in assignment (expression has type "None", variable has type "str")

test-data/unit/deps.test

@@ -516,7 +516,7 @@ class A:
 from n import A
 
 class B:
-    x = None  # type: A
+    x = None  # type: A  # type: ignore
 [file n.py]
 class A: pass
 [out]