Allow comparisons to refine Optional types without strict-optional

msullivan · msullivan · commit e77113a6fd8f · 2018-09-17T14:50:21.000-07:00
Fixes #4520.
diff --git a/mypy/checker.py b/mypy/checker.py
@@ -3278,7 +3278,7 @@ def find_isinstance_check(self, node: Expression
                 if literal(expr) == LITERAL_TYPE:
                     vartype = type_map[expr]
                     return self.conditional_callable_type_map(expr, vartype)
-        elif isinstance(node, ComparisonExpr) and experiments.STRICT_OPTIONAL:
+        elif isinstance(node, ComparisonExpr):
             # Check for `x is None` and `x is not None`.
             is_not = node.operators == ['is not']
             if any(is_literal_none(n) for n in node.operands) and (
diff --git a/test-data/unit/check-unions.test b/test-data/unit/check-unions.test
@@ -948,3 +948,21 @@ MYTYPE = List[Union[str, "MYTYPE"]]
 [builtins fixtures/list.pyi]
 [out]
 main:2: error: Recursive types not fully supported yet, nested types replaced with "Any"
+
+[case testNonStrictOptional]
+from typing import Optional, List
+
+def union_test1(x):
+    # type: (Optional[List[int]]) -> Optional[int]
+    if x is None:
+        return x
+    else:
+        return x[0]
+
+def union_test2(x):
+    # type: (Optional[List[int]]) -> Optional[int]
+    if isinstance(x, type(None)):
+        return x
+    else:
+        return x[0]
+[builtins fixtures/isinstancelist.pyi]
