diff --git a/jedi/evaluate/helpers.py b/jedi/evaluate/helpers.py
index 88a2fde8..dba8a868 100644
--- a/jedi/evaluate/helpers.py
+++ b/jedi/evaluate/helpers.py
@@ -5,11 +5,11 @@ from jedi.parser import representation as pr
 from jedi import debug
 
 
-def fast_parent_copy(obj):
+def fast_parent_copy(obj, new_elements_default=None):
     """
     Much, much faster than copy.deepcopy, but just for certain elements.
     """
-    new_elements = {}
+    new_elements = new_elements_default or {}
     accept = (pr.Simple, pr.NamePart)
 
     def recursion(obj):
@@ -18,8 +18,11 @@ def fast_parent_copy(obj):
             # correctly, don't know why.
             obj.get_defined_names()
 
-        new_obj = copy.copy(obj)
-        new_elements[obj] = new_obj
+        try:
+            return new_elements[obj]
+        except KeyError:
+            new_obj = copy.copy(obj)
+            new_elements[obj] = new_obj
 
         try:
             items = list(new_obj.__dict__.items())
@@ -48,10 +51,7 @@ def fast_parent_copy(obj):
             elif isinstance(value, (list, tuple)):
                 setattr(new_obj, key, list_or_tuple_rec(value))
             elif isinstance(value, accept):
-                try:
-                    setattr(new_obj, key, new_elements[value])
-                except KeyError:
-                    setattr(new_obj, key, recursion(value))
+                setattr(new_obj, key, recursion(value))
         return new_obj
 
     def list_or_tuple_rec(array_obj):
@@ -61,10 +61,7 @@ def fast_parent_copy(obj):
             copied_array = array_obj[:]   # lists, tuples, strings, unicode
         for i, el in enumerate(copied_array):
             if isinstance(el, accept):
-                try:
-                    copied_array[i] = new_elements[el]
-                except KeyError:
-                    copied_array[i] = recursion(el)
+                copied_array[i] = recursion(el)
             elif isinstance(el, (tuple, list)):
                 copied_array[i] = list_or_tuple_rec(el)
 
diff --git a/jedi/evaluate/representation.py b/jedi/evaluate/representation.py
index fb9d0fad..b10127fd 100644
--- a/jedi/evaluate/representation.py
+++ b/jedi/evaluate/representation.py
@@ -507,6 +507,11 @@ class FunctionExecution(Executed):
     multiple calls to functions and recursion has to be avoided. But this is
     responsibility of the decorators.
     """
+    def __init__(self, evaluator, base, *args, **kwargs):
+        super(FunctionExecution, self).__init__(evaluator, base, *args, **kwargs)
+        # for fast_parent_copy
+        self._copy_dict = {base.base_func: self}
+
     @memoize_default(default=())
     @recursion.execution_recursion_decorator
     def get_return_types(self):
@@ -571,13 +576,13 @@ class FunctionExecution(Executed):
         objects = []
         for element in attr:
             if element is None:
-                copied = element
+                copied = None
             else:
-                copied = helpers.fast_parent_copy(element)
-                copied.parent = self._scope_copy(copied.parent)
+                self._scope_copy(element.parent)
+                copied = helpers.fast_parent_copy(element, self._copy_dict)
                 # TODO remove? Doesn't make sense, at least explain.
-                if isinstance(copied, pr.Function):
-                    copied = Function(self._evaluator, copied)
+                #if isinstance(copied, pr.Function):
+                    #copied = Function(self._evaluator, copied)
             objects.append(copied)
         return objects
 
@@ -596,8 +601,9 @@ class FunctionExecution(Executed):
         if scope.start_pos == self.start_pos:
             return self
         else:
-            copied = helpers.fast_parent_copy(scope)
-            copied.parent = self._scope_copy(copied.parent)
+            self._scope_copy(scope.parent)
+            copied = helpers.fast_parent_copy(scope, self._copy_dict)
+            #copied.parent = self._scope_copy(copied.parent)
             return copied
 
     @common.safe_property
diff --git a/test/completion/isinstance.py b/test/completion/isinstance.py
index 0a08ca86..e44312f5 100644
--- a/test/completion/isinstance.py
+++ b/test/completion/isinstance.py
@@ -43,11 +43,12 @@ def fooooo(obj):
 
 
 def fooooo2(obj):
-    if isinstance(obj, datetime.datetime):
+    if isinstance(obj, datetime.date):
         return obj
     else:
         return 1
 
+a
 # In earlier versions of Jedi, this returned both datetime and int, but now
 # Jedi does flow checks and realizes that the top return isn't executed.
 #? int()