fix add_method_controller to deal properly in the event of exceptions.

add a docstring as well.

_pytest/core.py

@@ -67,20 +67,39 @@ class TagTracerSub:
     def get(self, name):
         return self.__class__(self.root, self.tags + (name,))
 
+
 def add_method_controller(cls, func):
+    """ Use func as the method controler for the method found
+    at the class named func.__name__.
+
+    A method controler is invoked with the same arguments
+    as the function it substitutes and is required to yield once
+    which will trigger calling the controlled method.
+    If it yields a second value, the value will be returned
+    as the result of the invocation.  Errors in the controlled function
+    are re-raised to the controller during the first yield.
+    """
     name = func.__name__
     oldcall = getattr(cls, name)
     def wrap_exec(*args, **kwargs):
         gen = func(*args, **kwargs)
         next(gen)   # first yield
-        res = oldcall(*args, **kwargs)
         try:
-            gen.send(res)
+            res = oldcall(*args, **kwargs)
+        except Exception:
+            excinfo = sys.exc_info()
+            try:
+                # reraise exception to controller
+                res = gen.throw(*excinfo)
+            except StopIteration:
+                py.builtin._reraise(*excinfo)
+        else:
+            try:
+                res = gen.send(res)
             except StopIteration:
                 pass
-        else:
-            raise ValueError("expected StopIteration")
         return res
+
     setattr(cls, name, wrap_exec)
     return lambda: setattr(cls, name, oldcall)
 

testing/test_core.py

@@ -765,8 +765,8 @@ def test_importplugin_issue375(testdir):
     assert "qwe" not in str(excinfo.value)
     assert "aaaa" in str(excinfo.value)
 
-
+class TestWrapMethod:
-def test_wrapping():
+    def test_basic_happypath(self):
         class A:
             def f(self):
                 return "A.f"
@@ -784,3 +784,77 @@ def test_wrapping():
         l[:] = []
         assert A().f() == "A.f"
         assert l == []
+
+    def test_method_raises(self):
+        class A:
+            def error(self, val):
+                raise ValueError(val)
+
+        l = []
+        def error(self, val):
+            l.append(val)
+            try:
+                yield
+            except ValueError:
+                l.append(None)
+                raise
+
+
+        undo = add_method_controller(A, error)
+
+        with pytest.raises(ValueError):
+            A().error(42)
+        assert l == [42, None]
+        undo()
+        l[:] = []
+        with pytest.raises(ValueError):
+            A().error(42)
+        assert l == []
+
+    def test_controller_swallows_method_raises(self):
+        class A:
+            def error(self, val):
+                raise ValueError(val)
+
+        def error(self, val):
+            try:
+                yield
+            except ValueError:
+                yield 2
+
+        add_method_controller(A, error)
+        assert A().error(42) == 2
+
+    def test_reraise_on_controller_StopIteration(self):
+        class A:
+            def error(self, val):
+                raise ValueError(val)
+
+        def error(self, val):
+            try:
+                yield
+            except ValueError:
+                pass
+
+        add_method_controller(A, error)
+        with pytest.raises(ValueError):
+            A().error(42)
+
+    @pytest.mark.xfail(reason="if needed later")
+    def test_modify_call_args(self):
+        class A:
+            def error(self, val1, val2):
+                raise ValueError(val1+val2)
+
+        l = []
+        def error(self):
+            try:
+                yield (1,), {'val2': 2}
+            except ValueError as ex:
+                assert ex.args == (3,)
+                l.append(1)
+
+        add_method_controller(A, error)
+        with pytest.raises(ValueError):
+            A().error()
+        assert l == [1]