Separate recursive importing from single node importing in FunctionGraph

aesara/graph/fg.py

@@ -311,7 +311,7 @@ def import_var(
 
             if isinstance(var.type, NullType):
                 raise TypeError(
-                    f"Computation graph contains a NaN. {var.type.why_null}"
+                    f"Computation graph contains a null type: {var} {var.type.why_null}"
                 )
             if import_missing:
                 self.add_input(var)
@@ -327,7 +327,7 @@ def import_node(
         reason: Optional[str] = None,
         import_missing: bool = False,
     ) -> None:
-        """Recursively import everything between an ``Apply`` node and the ``FunctionGraph``'s outputs.
+        """Recursively import everything between an `Apply` node and the `FunctionGraph`'s outputs.
 
         Parameters
         ----------
@@ -347,42 +347,62 @@ def import_node(
         # to know where to stop going down.)
         new_nodes = io_toposort(self.variables, apply_node.outputs)
 
-        if check:
-            for node in new_nodes:
-                for var in node.inputs:
-                    if (
-                        var.owner is None
-                        and not isinstance(var, AtomicVariable)
-                        and var not in self.inputs
-                    ):
-                        if import_missing:
-                            self.add_input(var)
-                        else:
-                            error_msg = (
-                                f"Input {node.inputs.index(var)} ({var})"
-                                " of the graph (indices start "
-                                f"from 0), used to compute {node}, was not "
-                                "provided and not given a value. Use the "
-                                "Aesara flag exception_verbosity='high', "
-                                "for more information on this error."
-                            )
-                            raise MissingInputError(error_msg, variable=var)
-
         for node in new_nodes:
-            assert node not in self.apply_nodes
-            self.apply_nodes.add(node)
-            if not hasattr(node.tag, "imported_by"):
-                node.tag.imported_by = []
-            node.tag.imported_by.append(str(reason))
-            for output in node.outputs:
-                self.setup_var(output)
-                self.variables.add(output)
-            for i, input in enumerate(node.inputs):
-                if input not in self.variables:
-                    self.setup_var(input)
-                    self.variables.add(input)
-                self.add_client(input, (node, i))
-            self.execute_callbacks("on_import", node, reason)
+            self._import_node(
+                node, check=check, reason=reason, import_missing=import_missing
+            )
+
+    def _import_node(
+        self,
+        apply_node: Apply,
+        check: bool = True,
+        reason: Optional[str] = None,
+        import_missing: bool = False,
+    ) -> None:
+        """Import a single node.
+
+        See `FunctionGraph.import_node`.
+        """
+        assert apply_node not in self.apply_nodes
+
+        for i, inp in enumerate(apply_node.inputs):
+            if (
+                check
+                and inp.owner is None
+                and not isinstance(inp, AtomicVariable)
+                and inp not in self.inputs
+            ):
+                if import_missing:
+                    self.add_input(inp)
+                else:
+                    error_msg = (
+                        f"Input {apply_node.inputs.index(inp)} ({inp})"
+                        " of the graph (indices start "
+                        f"from 0), used to compute {apply_node}, was not "
+                        "provided and not given a value. Use the "
+                        "Aesara flag exception_verbosity='high', "
+                        "for more information on this error."
+                    )
+                    raise MissingInputError(error_msg, variable=inp)
+
+            if inp not in self.variables:
+                self.setup_var(inp)
+                self.variables.add(inp)
+
+            self.add_client(inp, (apply_node, i))
+
+        for output in apply_node.outputs:
+            self.setup_var(output)
+            self.variables.add(output)
+
+        self.apply_nodes.add(apply_node)
+
+        if not hasattr(apply_node.tag, "imported_by"):
+            apply_node.tag.imported_by = []
+
+        apply_node.tag.imported_by.append(str(reason))
+
+        self.execute_callbacks("on_import", apply_node, reason)
 
     def change_node_input(
         self,