Process try statements in top-to-bottom order

Fixes #1289 (again).

Author: rwbarton

mypy/checker.py

@@ -1543,44 +1543,51 @@ def visit_try_stmt(self, s: TryStmt) -> Type:
     def visit_try_without_finally(self, s: TryStmt) -> None:
         """Type check a try statement, ignoring the finally block.
 
-        Otherwise, it will place the results possible frames of
-        that don't break out into self.binder.frames[-2].
+        On entry, the top frame should receive all flow that exits the
+        try block abnormally (i.e., such that the else block does not
+        execute), and its parent should receive all flow that exits
+        the try block normally.
         """
-        # This frame records the possible states that exceptions can leave variables in
-        # during the try: block
-        with self.binder.frame_context(can_skip=False, fall_through=0):
-            with self.binder.frame_context(can_skip=False, fall_through=3):
-                self.binder.try_frames.add(len(self.binder.frames) - 2)
-                self.binder.allow_jump(-1)
-                self.accept(s.body)
-                self.binder.try_frames.remove(len(self.binder.frames) - 2)
-                if s.else_body:
-                    self.accept(s.else_body)
-            for i in range(len(s.handlers)):
-                with self.binder.frame_context(can_skip=True, fall_through=3):
-                    if s.types[i]:
-                        t = self.visit_except_handler_test(s.types[i])
+        # This frame will run the else block if the try fell through.
+        # In that case, control flow continues to the parent of what
+        # was the top frame on entry.
+        with self.binder.frame_context(can_skip=False, fall_through=2):
+            # This frame receives exit via exception, and runs exception handlers
+            with self.binder.frame_context(can_skip=False, fall_through=2):
+                # Finally, the body of the try statement
+                with self.binder.frame_context(can_skip=False, fall_through=2):
+                    self.binder.try_frames.add(len(self.binder.frames) - 2)
+                    self.binder.allow_jump(-1)
+                    self.accept(s.body)
+                    self.binder.try_frames.remove(len(self.binder.frames) - 2)
+                for i in range(len(s.handlers)):
+                    with self.binder.frame_context(can_skip=True, fall_through=4):
+                        if s.types[i]:
+                            t = self.visit_except_handler_test(s.types[i])
+                            if s.vars[i]:
+                                # To support local variables, we make this a definition line,
+                                # causing assignment to set the variable's type.
+                                s.vars[i].is_def = True
+                                self.check_assignment(s.vars[i], self.temp_node(t, s.vars[i]))
+                        self.accept(s.handlers[i])
                         if s.vars[i]:
-                            # To support local variables, we make this a definition line,
-                            # causing assignment to set the variable's type.
-                            s.vars[i].is_def = True
-                            self.check_assignment(s.vars[i], self.temp_node(t, s.vars[i]))
-                    self.accept(s.handlers[i])
-                    if s.vars[i]:
-                        # Exception variables are deleted in python 3 but not python 2.
-                        # But, since it's bad form in python 2 and the type checking
-                        # wouldn't work very well, we delete it anyway.
-
-                        # Unfortunately, this doesn't let us detect usage before the
-                        # try/except block.
-                        if self.pyversion[0] >= 3:
-                            source = s.vars[i].name
-                        else:
-                            source = ('(exception variable "{}", which we do not accept outside'
-                                      'except: blocks even in python 2)'.format(s.vars[i].name))
-                        var = cast(Var, s.vars[i].node)
-                        var.type = DeletedType(source=source)
-                        self.binder.cleanse(s.vars[i])
+                            # Exception variables are deleted in python 3 but not python 2.
+                            # But, since it's bad form in python 2 and the type checking
+                            # wouldn't work very well, we delete it anyway.
+
+                            # Unfortunately, this doesn't let us detect usage before the
+                            # try/except block.
+                            if self.pyversion[0] >= 3:
+                                source = s.vars[i].name
+                            else:
+                                source = ('(exception variable "{}", which we do not '
+                                          'accept outside except: blocks even in '
+                                          'python 2)'.format(s.vars[i].name))
+                            var = cast(Var, s.vars[i].node)
+                            var.type = DeletedType(source=source)
+                            self.binder.cleanse(s.vars[i])
+            if s.else_body:
+                self.accept(s.else_body)
 
     def visit_except_handler_test(self, n: Node) -> Type:
         """Type check an exception handler test clause."""

test-data/unit/check-statements.test

@@ -516,7 +516,7 @@ else:
   object(None) # E: Too many arguments for "object"
 [builtins fixtures/exception.py]
 
-[case testRedefinedFunctionInTryWithElse-skip]
+[case testRedefinedFunctionInTryWithElse]
 def f() -> None: pass
 try:
     pass