Unsuccessful attempt to graph observations.

I don't have a clear enough notion of what can be in the `observations` property of a random variable.

pymc3/model_graph.py

@@ -1,5 +1,5 @@
 from collections import deque
-from typing import Iterator, Optional, MutableSet
+from typing import Iterator, Optional, MutableSet, FrozenSet, Tuple
 
 from theano.gof.graph import stack_search
 from theano.compile import SharedVariable
@@ -34,7 +34,7 @@ def _get_ancestors(self, var, func) -> MutableSet[Tensor]:
         vars = set(self.var_list)
         vars.remove(var)
 
-        blockers = set()
+        blockers = set() # type: MutableSet[Tensor]
         retval = set()
         def _expand(node) -> Optional[Iterator[Tensor]]:
             if node in blockers:
@@ -54,9 +54,9 @@ def _expand(node) -> Optional[Iterator[Tensor]]:
                      mode='bfs')
         return retval
 
-    def _filter_parents(self, var, parents):
+    def _filter_parents(self, var, parents) -> Tuple[FrozenSet[str], FrozenSet[str]]:
         """Get direct parents of a var, as strings"""
-        keep = set()
+        keep = set() # type: MutableSet[str]
         for p in parents:
             if p == var:
                 continue
@@ -67,9 +67,14 @@ def _filter_parents(self, var, parents):
                     keep.add(self.transform_map[p])
             else:
                 raise AssertionError('Do not know what to do with {}'.format(str(p)))
-        return keep
+        children = frozenset() # type: FrozenSet[str]
+        try:
+            children = frozenset(var.observations.name)
+        except AttributeError:
+            pass
+        return frozenset(keep - children), children
 
-    def get_parents(self, var):
+    def get_parents(self, var) -> Tuple[FrozenSet[str], FrozenSet[str]]:
         """Get the named nodes that are direct inputs to the var"""
         if hasattr(var, 'transformed'):
             func = var.transformed.logpt
@@ -83,9 +88,18 @@ def get_parents(self, var):
 
     def make_compute_graph(self):
         """Get map of var_name -> set(input var names) for the model"""
-        input_map = {}
+        input_map = {} # type: Dict[str, FrozenSet[str]]
         for var_name in self.var_names:
-            input_map[var_name] = self.get_parents(self.model[var_name])
+            # the "children" here are exclusively observations
+            parents, children = self.get_parents(self.model[var_name])
+            input_map[var_name] = parents
+            for child in children:
+                if child in input_map:
+                    # slightly awkward union because input map values are not
+                    # mutable
+                    input_map[child] = frozenset(input_map[child] + set(var_name))
+                else:
+                    input_map[child] = frozenset(var_name)
         return input_map
 
     def _make_node(self, var_name, graph):