Fix contextmanager transform for nested contextmanagers

Close PyCQA/pylint#1746

ChangeLog

@@ -18,6 +18,11 @@ Change log for the astroid package (used to be astng)
 	Close PyCQA/pylint#1884
+    * Fix ``contextlib.contextmanager`` inference for nested
+        context managers
+
+        Close #1699
+
 2018-01-23 -- 1.6.1

astroid/protocols.py

@@ -445,7 +445,10 @@ def _infer_context_manager(self, mgr, context):
         # Get the first yield point. If it has multiple yields,
         # then a RuntimeError will be raised.
         # TODO(cpopa): Handle flows.
-        yield_point = next(func.nodes_of_class(nodes.Yield), None)
+        possible_yield_points = func.nodes_of_class(nodes.Yield)
+        # Ignore yields in nested functions
+        yield_point = next((node for node in possible_yield_points
+                            if node.scope() == func), None)
         if yield_point:
             if not yield_point.value:
                 # TODO(cpopa): an empty yield. Should be wrapped to Const.

astroid/tests/unittest_inference.py

@@ -2154,6 +2154,36 @@ class InferenceTest(resources.SysPathSetup, unittest.TestCase):
         self.assertRaises(InferenceError, next, module['other_decorators'].infer())
         self.assertRaises(InferenceError, next, module['no_yield'].infer())
+    def test_nested_contextmanager(self):
+        """Make sure contextmanager works with nested functions
+
+        Previously contextmanager would retrieve
+        the first yield instead of the yield in the
+        proper scope
+
+        Fixes https://github.com/PyCQA/pylint/issues/1746
+        """
+        code = """
+        from contextlib import contextmanager
+
+        @contextmanager
+        def outer():
+            @contextmanager
+            def inner():
+                yield 2
+            yield inner
+
+        with outer() as ctx:
+            ctx #@
+            with ctx() as val:
+                val #@
+        """
+        context_node, value_node = extract_node(code)
+        value = next(value_node.infer())
+        context = next(context_node.infer())
+        assert isinstance(context, nodes.FunctionDef)
+        assert isinstance(value, nodes.Const)
+
     def test_unary_op_leaks_stop_iteration(self):
         node = extract_node('+[] #@')
         self.assertEqual(util.Uninferable, next(node.infer()))