Add a way how dict setitem can be understood

Needs the latest parso commits

jedi/inference/value/dynamic_arrays.py

@@ -22,10 +22,14 @@ It is important to note that:
 from jedi import debug
 from jedi import settings
 from jedi.inference import recursion
-from jedi.inference.base_value import ValueSet, NO_VALUES, HelperValueMixin
+from jedi.inference.base_value import ValueSet, NO_VALUES, HelperValueMixin, \
+    ValueWrapper
+from jedi.inference.lazy_value import LazyKnownValues
 from jedi.inference.helpers import infer_call_of_leaf
 from jedi.inference.cache import inference_state_method_cache
 
+_sentinel = object()
+
 
 def check_array_additions(context, sequence):
     """ Just a mapper function for the internal _internal_check_array_additions """
@@ -162,3 +166,36 @@ class _ArrayInstance(HelperValueMixin):
 
     def iterate(self, contextualized_node=None, is_async=False):
         return self.py__iter__(contextualized_node)
+
+
+class _Modification(ValueWrapper):
+    def __init__(self, wrapped_value, assigned_values, contextualized_key):
+        super(_Modification, self).__init__(wrapped_value)
+        self._assigned_values = assigned_values
+        self._contextualized_key = contextualized_key
+
+    def py__getitem__(self, *args, **kwargs):
+        return self._wrapped_value.py__getitem__(*args, **kwargs) | self._assigned_values
+
+    def py__simple_getitem__(self, index):
+        actual = [
+            v.get_safe_value(_sentinel)
+            for v in self._contextualized_key.infer()
+        ]
+        if index in actual:
+            return self._assigned_values
+        return self._wrapped_value.py__simple_getitem__(index)
+
+
+class DictModification(_Modification):
+    def py__iter__(self):
+        for lazy_context in self._wrapped_value.py__iter__():
+            yield lazy_context
+        yield self._contextualized_key
+
+
+class ListModification(_Modification):
+    def py__iter__(self):
+        for lazy_context in self._wrapped_value.py__iter__():
+            yield lazy_context
+        yield LazyKnownValues(self._assigned_values)