WIP

hamilton/graph.py

diff --git a/hamilton/graph.py b/hamilton/graph.py
index c4d9c6e..34ba41a 100644
--- a/hamilton/graph.py
+++ b/hamilton/graph.py
@@ -520,6 +520,21 @@
     return digraph
 
 
+def _validate_all_nodes(adapter: LifecycleAdapterSet, nodes: Dict[str, node.Node]):
+    invalid_nodes_with_errors = []
+    if adapter.does_method("do_validate_node", is_async=False):
+        for n in nodes.values():
+            is_valid, error = adapter.call_lifecycle_method_sync("do_validate_node", node=n)
+            if not is_valid:
+                invalid_nodes_with_errors.append((n, error))
+
+    if invalid_nodes_with_errors:
+        raise ValueError(
+            "The following nodes are invalid:\n"
+            + "\n".join([f"{n.name} : {error}" for n, error in invalid_nodes_with_errors])
+        )
+
+
 class FunctionGraph:
     """Note: this object should be considered private until stated otherwise.
 
@@ -542,6 +557,8 @@
         if adapter is None:
             adapter = LifecycleAdapterSet(base.SimplePythonDataFrameGraphAdapter())
 
+        _validate_all_nodes(adapter, nodes)
+
         self._config = config
         self.nodes = nodes
         self.adapter = adapter

hamilton/lifecycle/api.py

diff --git a/hamilton/lifecycle/api.py b/hamilton/lifecycle/api.py
index 8a202ec..208de5b 100644
--- a/hamilton/lifecycle/api.py
+++ b/hamilton/lifecycle/api.py
@@ -1,6 +1,6 @@
 import abc
 from abc import ABC
-from typing import Any, Dict, List, Optional, Type
+from typing import Any, Dict, List, Optional, Tuple, Type
 
 from hamilton import node
 from hamilton.lifecycle.base import (
@@ -8,9 +8,11 @@
     BaseDoCheckEdgeTypesMatch,
     BaseDoNodeExecute,
     BaseDoValidateInput,
+    BaseDoValidateNode,
     BasePostNodeExecute,
     BasePreNodeExecute,
 )
+from hamilton.node import DependencyType
 
 try:
     from typing import override
@@ -307,3 +309,53 @@
         :return: The result of the node execution -- up to you to return this.
         """
         pass
+
+
+class NodeValidationMethod(BaseDoValidateNode):
+    def do_validate_node(self, *, created_node: node.Node) -> Tuple[bool, Optional[str]]:
+        return self.validate_node(
+            node_name=created_node.name,
+            node_module=created_node.tags.get("module", None),
+            node_tags=created_node.tags,
+            required_dependencies=[
+                item
+                for item, dep_type in created_node.input_types.items()
+                if dep_type == DependencyType.REQUIRED
+            ],
+            optional_dependencies=[
+                item
+                for item, dep_type in created_node.input_types.items()
+                if dep_type == DependencyType.OPTIONAL
+            ],
+            node_type=created_node.type,
+        )
+
+    def validate_node(
+        self,
+        *,
+        node_name: str,
+        node_module: Optional[str],
+        node_tags: Dict[str, str],
+        required_dependencies: List[str],
+        optional_dependencies: List[str],
+        node_type: Type,
+        **kwargs: Any,
+    ) -> Tuple[bool, Optional[str]]:
+        """Validate a node. You have access to tags, types, etc...
+        We also reserve the right to add future kwargs. This is after node creation,
+        during graph construction.
+
+        Note that this method allows you to raise an InvalidNodeException if you want to
+        stop the graph construction. This is useful if you want to do some validation
+        on tags, for instance.
+
+        :param node_name: Name of the node in question
+        :param node_module: Module of the function that defined the node, if we know it
+        :param node_tags: Tags of the node
+        :param required_dependencies: List of required dependencies for the node
+        :param optional_dependencies: List of optional dependencies for the node
+        :param node_type: Return type of the node
+        :param kwargs: Keyword arguments -- this is kept for future backwards compatibility.
+        :return: Whether or not the node is valid, and an optional error message
+        """
+        pass

hamilton/lifecycle/base.py

diff --git a/hamilton/lifecycle/base.py b/hamilton/lifecycle/base.py
index 0c900b7..40c3fef 100644
--- a/hamilton/lifecycle/base.py
+++ b/hamilton/lifecycle/base.py
@@ -199,7 +199,7 @@
 @lifecycle.base_method("do_validate_node")
 class BaseDoValidateNode(abc.ABC):
     @abc.abstractmethod
-    def do_validate_node(self, *, created_node: node.Node) -> bool:
+    def do_validate_node(self, *, created_node: node.Node) -> Tuple[bool, Optional[str]]:
         """Validates a node. Note this is *not* integrated yet, so adding this in will be a No-op.
         In fact, we will likely be changing the API for this to have an optional error message.
         This is OK, as this is internal facing.
@@ -207,7 +207,7 @@
         Furthermore, we'll be adding in a user-facing API that takes in the tags, name, module, etc...
 
         :param created_node: Node that was created.
-        :return: Whether or not the node is valid.
+        :return: Whether or not the node is valid, and, if it is not, an error message
         """
         pass