Explain why two try frames are needed for a finally clause; add a test

Author: rwbarton

mypy/checker.py

@@ -1640,10 +1640,15 @@ def visit_try_stmt(self, s: TryStmt) -> Type:
         # (by exception, return, break, etc.)
         with self.binder.frame_context(can_skip=False, fall_through=0):
             if s.finally_body:
+                # Not only might the body of the try statement exit abnormally,
+                # but so might an exception handler or else clause. The finally
+                # clause runs in *all* cases, so we need an outer try frame to
+                # catch all intermediate states in case an exception is raised
+                # during an except or else clause.
                 self.binder.try_frames.add(len(self.binder.frames) - 1)
                 self.visit_try_without_finally(s)
                 self.binder.try_frames.remove(len(self.binder.frames) - 1)
-                # First we check finally_body is type safe for all intermediate frames
+                # First we check finally_body is type safe on all abnormal exit paths
                 self.accept(s.finally_body)
             else:
                 self.visit_try_without_finally(s)

test-data/unit/check-isinstance.test

@@ -352,6 +352,22 @@ while 2:
         break
         x.b
     x.b
+[case testUnionTryFinally6]
+class A: pass
+class B(A): b = 1
+
+def f() -> int:
+    x = B()  # type: A
+    try:
+        x = B()
+    except:
+        x = A()
+        # An exception could occur here
+        x = B()
+    finally:
+        return x.b # E: "A" has no attribute "b"
+[out]
+main: note: In function "f":
 [case testUnionListIsinstance]
 
 from typing import Union, List