implement a new hook type: hook wrappers using a "yield" to distinguish

between working at the front and at the end of a hook call chain.
The idea is to make it easier for a plugin to "wrap" a certain hook
call and use context managers, in particular allow a major cleanup of
capturing.

_pytest/core.py

@@ -240,18 +240,22 @@ class PluginManager(object):
             pass
         l = []
         last = []
+        wrappers = []
         for plugin in plugins:
             try:
                 meth = getattr(plugin, attrname)
-                if hasattr(meth, 'tryfirst'):
+            except AttributeError:
+                continue
+            if hasattr(meth, 'hookwrapper'):
+                wrappers.append(meth)
+            elif hasattr(meth, 'tryfirst'):
                 last.append(meth)
             elif hasattr(meth, 'trylast'):
                 l.insert(0, meth)
             else:
                 l.append(meth)
-            except AttributeError:
-                continue
         l.extend(last)
+        l.extend(wrappers)
         self._listattrcache[key] = list(l)
         return l
 
@@ -272,6 +276,14 @@ def importplugin(importspec):
 
 class MultiCall:
     """ execute a call into multiple python functions/methods. """
+
+    class WrongHookWrapper(Exception):
+        """ a hook wrapper does not behave correctly. """
+        def __init__(self, func, message):
+            Exception.__init__(self, func, message)
+            self.func = func
+            self.message = message
+
     def __init__(self, methods, kwargs, firstresult=False):
         self.methods = list(methods)
         self.kwargs = kwargs
@@ -283,9 +295,22 @@ class MultiCall:
         return "<MultiCall %s, kwargs=%r>" %(status, self.kwargs)
 
     def execute(self):
+        next_finalizers = []
+        try:
             while self.methods:
                 method = self.methods.pop()
                 kwargs = self.getkwargs(method)
+                if hasattr(method, "hookwrapper"):
+                    it = method(**kwargs)
+                    next = getattr(it, "next", None)
+                    if next is None:
+                        next = getattr(it, "__next__", None)
+                        if next is None:
+                            raise self.WrongHookWrapper(method,
+                                "wrapper does not contain a yield")
+                    res = next()
+                    next_finalizers.append((method, next))
+                else:
                     res = method(**kwargs)
                 if res is not None:
                     self.results.append(res)
@@ -293,6 +318,16 @@ class MultiCall:
                         return res
             if not self.firstresult:
                 return self.results
+        finally:
+            for method, fin in reversed(next_finalizers):
+                try:
+                    fin()
+                except StopIteration:
+                    pass
+                else:
+                    raise self.WrongHookWrapper(method,
+                                "wrapper contain more than one yield")
+
 
     def getkwargs(self, method):
         kwargs = {}

testing/test_core.py

@@ -523,6 +523,95 @@ class TestMultiCall:
         res = MultiCall([m1, m2], {}).execute()
         assert res == [1]
 
+    def test_hookwrapper(self):
+        l = []
+        def m1():
+            l.append("m1 init")
+            yield None
+            l.append("m1 finish")
+        m1.hookwrapper = True
+
+        def m2():
+            l.append("m2")
+            return 2
+        res = MultiCall([m2, m1], {}).execute()
+        assert res == [2]
+        assert l == ["m1 init", "m2", "m1 finish"]
+        l[:] = []
+        res = MultiCall([m2, m1], {}, firstresult=True).execute()
+        assert res == 2
+        assert l == ["m1 init", "m2", "m1 finish"]
+
+    def test_hookwrapper_order(self):
+        l = []
+        def m1():
+            l.append("m1 init")
+            yield 1
+            l.append("m1 finish")
+        m1.hookwrapper = True
+
+        def m2():
+            l.append("m2 init")
+            yield 2
+            l.append("m2 finish")
+        m2.hookwrapper = True
+        res = MultiCall([m2, m1], {}).execute()
+        assert res == [1, 2]
+        assert l == ["m1 init", "m2 init", "m2 finish", "m1 finish"]
+
+    def test_listattr_hookwrapper_ordering(self):
+        class P1:
+            @pytest.mark.hookwrapper
+            def m(self):
+                return 17
+
+        class P2:
+            def m(self):
+                return 23
+
+        class P3:
+            @pytest.mark.tryfirst
+            def m(self):
+                return 19
+
+        pluginmanager = PluginManager()
+        p1 = P1()
+        p2 = P2()
+        p3 = P3()
+        pluginmanager.register(p1)
+        pluginmanager.register(p2)
+        pluginmanager.register(p3)
+        methods = pluginmanager.listattr('m')
+        assert methods == [p2.m, p3.m, p1.m]
+        ## listattr keeps a cache and deleting
+        ## a function attribute requires clearing it
+        #pluginmanager._listattrcache.clear()
+        #del P1.m.__dict__['tryfirst']
+
+    def test_hookwrapper_not_yield(self):
+        def m1():
+            pass
+        m1.hookwrapper = True
+
+        mc = MultiCall([m1], {})
+        with pytest.raises(mc.WrongHookWrapper) as ex:
+            mc.execute()
+        assert ex.value.func == m1
+        assert ex.value.message
+
+    def test_hookwrapper_too_many_yield(self):
+        def m1():
+            yield 1
+            yield 2
+        m1.hookwrapper = True
+
+        mc = MultiCall([m1], {})
+        with pytest.raises(mc.WrongHookWrapper) as ex:
+            mc.execute()
+        assert ex.value.func == m1
+        assert ex.value.message
+
+
 class TestHookRelay:
     def test_happypath(self):
         pm = PluginManager()