some docs and refined semantics for wrappers

_pytest/core.py

@@ -89,30 +89,34 @@ def add_method_wrapper(cls, wrapper_func):
     setattr(cls, name, wrap_exec)
     return lambda: setattr(cls, name, oldcall)
 
+def raise_wrapfail(wrap_controller, msg):
+    co = wrap_controller.gi_code
+    raise RuntimeError("wrap_controller at %r %s:%d %s" %
+                   (co.co_name, co.co_filename, co.co_firstlineno, msg))
 
 def wrapped_call(wrap_controller, func):
-    """ Wrap calling to a function with a generator.  The first yield
-    will trigger calling the function and receive an according CallOutcome
-    object representing an exception or a result.  The generator then
-    needs to finish (raise StopIteration) in order for the wrapped call
-    to complete.
+    """ Wrap calling to a function with a generator which needs to yield
+    exactly once.  The yield point will trigger calling the wrapped function
+    and return its CallOutcome to the yield point.  The generator then needs
+    to finish (raise StopIteration) in order for the wrapped call to complete.
     """
     try:
         next(wrap_controller)   # first yield
     except StopIteration:
-        return
+        raise_wrapfail(wrap_controller, "did not yield")
     call_outcome = CallOutcome(func)
     try:
         wrap_controller.send(call_outcome)
-        co = wrap_controller.gi_frame.f_code
-        raise RuntimeError("wrap_controller for %r %s:%d has second yield" %
-                           (co.co_name, co.co_filename, co.co_firstlineno))
+        raise_wrapfail(wrap_controller, "has second yield")
     except StopIteration:
         pass
     return call_outcome.get_result()
 
 
 class CallOutcome:
+    """ Outcome of a function call, either an exception or a proper result.
+    Calling the ``get_result`` method will return the result or reraise
+    the exception raised when the function was called. """
     excinfo = None
     def __init__(self, func):
         try:

doc/en/plugins.txt

@@ -187,8 +187,8 @@ Plugin discovery order at tool startup
   invocation:
 
   - if no test paths are specified use current dir as a test path
-  - if exists, load ``conftest.py`` and ``test*/conftest.py`` relative 
-    to the directory part of the first test path. 
+  - if exists, load ``conftest.py`` and ``test*/conftest.py`` relative
+    to the directory part of the first test path.
 
   Note that pytest does not find ``conftest.py`` files in deeper nested
   sub directories at tool startup.  It is usually a good idea to keep
@@ -431,6 +431,41 @@ declaring the hook functions directly in your plugin module, for example::
 This has the added benefit of allowing you to conditionally install hooks
 depending on which plugins are installed.
 
+hookwrapper: executing around other hooks
+-------------------------------------------------
+
+.. currentmodule:: _pytest.core
+
+.. versionadded:: 2.7 (experimental)
+
+pytest plugins can implement hook wrappers which which wrap the execution
+of other hook implementations.  A hook wrapper is a generator function
+which yields exactly once. When pytest invokes hooks it first executes
+hook wrappers and passes the same arguments as to the regular hooks.
+
+At the yield point of the hook wrapper pytest will execute the next hook
+implementations and return their result to the yield point in the form of
+a :py:class:`CallOutcome` instance which encapsulates a result or
+exception info.  The yield point itself will thus typically not raise
+exceptions (unless there are bugs).
+
+Here is an example definition of a hook wrapper::
+
+    import pytest
+
+    @pytest.mark.hookwrapper
+    def pytest_pyfunc_call(pyfuncitem):
+        # do whatever you want before the next hook executes
+        outcome = yield
+        # outcome.excinfo may be None or a (cls, val, tb) tuple
+        res = outcome.get_result()  # will raise if outcome was exception
+        # postprocess result
+
+Note that hook wrappers don't return results themselves, they merely
+perform tracing or other side effects around the actual hook implementations.
+If the result of the underlying hook is a mutable object, they may modify
+that result, however.
+
 
 Reference of objects involved in hooks
 ===========================================================
@@ -470,3 +505,6 @@ Reference of objects involved in hooks
 .. autoclass:: _pytest.runner.TestReport()
     :members:
 
+.. autoclass:: _pytest.core.CallOutcome()
+    :members:
+

testing/test_core.py

@@ -783,6 +783,22 @@ class TestWrapMethod:
         assert A().f() == "A.f"
         assert l == []
 
+    def test_no_yield(self):
+        class A:
+            def method(self):
+                return
+
+        def method(self):
+            if 0:
+                yield
+
+        add_method_wrapper(A, method)
+        with pytest.raises(RuntimeError) as excinfo:
+            A().method()
+
+        assert "method" in str(excinfo.value)
+        assert "did not yield" in str(excinfo.value)
+
     def test_method_raises(self):
         class A:
             def error(self, val):