Fix aststrip deletion crash (python#7543)

msullivan · web-flow · commit 959eb7613c59 · 2019-09-21T09:53:30.000-07:00
Only delete from self.type.names if the key is actually in there. I've included one test case that this fixes, though I suspect there are others. Fixes python#7238.
diff --git a/mypy/server/aststrip.py b/mypy/server/aststrip.py
@@ -219,7 +219,8 @@ def process_lvalue_in_method(self, lvalue: Node) -> None:
                 # true for a MemberExpr, we know that it must be an assignment through
                 # self, since only those can define new attributes.
                 assert self.type is not None
-                del self.type.names[lvalue.name]
+                if lvalue.name in self.type.names:
+                    del self.type.names[lvalue.name]
                 key = (self.type.defn, lvalue.name)
                 if key in self.saved_class_attrs:
                     del self.saved_class_attrs[key]
diff --git a/test-data/unit/fine-grained.test b/test-data/unit/fine-grained.test
@@ -9202,3 +9202,25 @@ class A: # type: ignore
 
 [out]
 ==
+
+[case testAddAttributeThroughNewBaseClass]
+import a
+
+[file a.py]
+class C:
+    def __init__(self) -> None:
+        self.x = 0
+
+[file a.py.2]
+from b import B
+
+class C(B):
+    def __init__(self) -> None:
+        self.x = 0
+
+[file b.py.2]
+class B:
+    def __init__(self) -> None:
+        self.x = 0
+[out]
+==
