Fix issues with yield.

jedi/evaluate/context.py

@@ -5,7 +5,10 @@ from jedi.parser_utils import clean_scope_docstring, get_doc_with_call_signature
 
 
 class Context(object):
-    api_type = None
+    """
+    Should be defined, otherwise the API returns empty types.
+    """
+
     """
     To be defined by subclasses.
     """
@@ -16,6 +19,12 @@ class Context(object):
         self.evaluator = evaluator
         self.parent_context = parent_context
 
+    @property
+    def api_type(self):
+        # By default just lower name of the class. Can and should be
+        # overwritten.
+        return self.__class__.__name__.lower()
+
     def get_root_context(self):
         context = self
         while True:

jedi/evaluate/iterable.py

@@ -165,7 +165,6 @@ class GeneratorMixin(object):
 
 class Generator(GeneratorMixin, context.Context):
     """Handling of `yield` functions."""
-
     def __init__(self, evaluator, func_execution_context):
         super(Generator, self).__init__(evaluator, parent_context=evaluator.BUILTINS)
         self._func_execution_context = func_execution_context

jedi/evaluate/representation.py

@@ -342,6 +342,11 @@ class FunctionExecutionContext(context.TreeContext):
         return types
 
     def _eval_yield(self, yield_expr):
+        if yield_expr.type == 'keyword':
+            # `yield` just yields None.
+            yield context.LazyKnownContext(compiled.create(self.evaluator, None))
+            return
+
         node = yield_expr.children[1]
         if node.type == 'yield_arg':  # It must be a yield from.
             cn = ContextualizedNode(self, node.children[1])

test/completion/generators.py

@@ -179,6 +179,25 @@ gen().send()
 #? 
 gen()()
 
+# -----------------
+# empty yield
+# -----------------
+
+def x():
+    yield
+
+#? None
+next(x())
+#? gen()
+x()
+
+def x():
+    for i in range(3):
+        yield
+
+#? None
+next(x())
+
 # -----------------
 # yield in expression
 # -----------------