Fix loading of ImagePromptTemplate (#16868)

We didn't override the namespace of the ImagePromptTemplate, so it is
listed as being in langchain.schema

This updates the mapping to let the loader deserialize.

Alternatively, we could make a slight breaking change and update the
namespace of the ImagePromptTemplate since we haven't broadly
publicized/documented it yet..

libs/core/langchain_core/load/mapping.py

@@ -115,6 +115,12 @@ SERIALIZABLE_MAPPING: Dict[Tuple[str, ...], Tuple[str, ...]] = {
         "chat",
         "SystemMessagePromptTemplate",
     ),
+    ("langchain", "prompts", "image", "ImagePromptTemplate"): (
+        "langchain_core",
+        "prompts",
+        "image",
+        "ImagePromptTemplate",
+    ),
     ("langchain", "schema", "agent", "AgentActionMessageLog"): (
         "langchain_core",
         "agents",
@@ -510,6 +516,12 @@ _OG_SERIALIZABLE_MAPPING: Dict[Tuple[str, ...], Tuple[str, ...]] = {
         "system",
         "SystemMessage",
     ),
+    ("langchain", "schema", "prompt_template", "ImagePromptTemplate"): (
+        "langchain_core",
+        "prompts",
+        "image",
+        "ImagePromptTemplate",
+    ),
 }
 
 # Needed for backwards compatibility for a few versions where we serialized

libs/core/langchain_core/prompts/image.py

@@ -1,4 +1,4 @@
-from typing import Any
+from typing import Any, List
 
 from langchain_core.prompt_values import ImagePromptValue, ImageURL, PromptValue
 from langchain_core.prompts.base import BasePromptTemplate
@@ -30,6 +30,11 @@ class ImagePromptTemplate(BasePromptTemplate[ImageURL]):
         """Return the prompt type key."""
         return "image-prompt"
 
+    @classmethod
+    def get_lc_namespace(cls) -> List[str]:
+        """Get the namespace of the langchain object."""
+        return ["langchain", "prompts", "image"]
+
     def format_prompt(self, **kwargs: Any) -> PromptValue:
         """Create Chat Messages."""
         return ImagePromptValue(image_url=self.format(**kwargs))

libs/core/tests/unit_tests/prompts/test_image.py

@@ -0,0 +1,109 @@
+import json
+
+from langchain_core.load import dump, loads
+from langchain_core.prompts import ChatPromptTemplate
+
+
+def test_image_prompt_template_deserializable() -> None:
+    """Test that the image prompt template is serializable."""
+    loads(
+        dump.dumps(
+            ChatPromptTemplate.from_messages(
+                [("system", [{"type": "image", "image_url": "{img}"}])]
+            )
+        )
+    )
+
+
+def test_image_prompt_template_deserializable_old() -> None:
+    """Test that the image prompt template is serializable."""
+    loads(
+        json.dumps(
+            {
+                "lc": 1,
+                "type": "constructor",
+                "id": ["langchain", "prompts", "chat", "ChatPromptTemplate"],
+                "kwargs": {
+                    "messages": [
+                        {
+                            "lc": 1,
+                            "type": "constructor",
+                            "id": [
+                                "langchain",
+                                "prompts",
+                                "chat",
+                                "SystemMessagePromptTemplate",
+                            ],
+                            "kwargs": {
+                                "prompt": [
+                                    {
+                                        "lc": 1,
+                                        "type": "constructor",
+                                        "id": [
+                                            "langchain",
+                                            "prompts",
+                                            "prompt",
+                                            "PromptTemplate",
+                                        ],
+                                        "kwargs": {
+                                            "template": "Foo",
+                                            "input_variables": [],
+                                            "template_format": "f-string",
+                                            "partial_variables": {},
+                                        },
+                                    }
+                                ]
+                            },
+                        },
+                        {
+                            "lc": 1,
+                            "type": "constructor",
+                            "id": [
+                                "langchain",
+                                "prompts",
+                                "chat",
+                                "HumanMessagePromptTemplate",
+                            ],
+                            "kwargs": {
+                                "prompt": [
+                                    {
+                                        "lc": 1,
+                                        "type": "constructor",
+                                        "id": [
+                                            "langchain",
+                                            "prompts",
+                                            "image",
+                                            "ImagePromptTemplate",
+                                        ],
+                                        "kwargs": {
+                                            "template": {
+                                                "url": "data:image/png;base64,{img}"
+                                            },
+                                            "input_variables": ["img"],
+                                        },
+                                    },
+                                    {
+                                        "lc": 1,
+                                        "type": "constructor",
+                                        "id": [
+                                            "langchain",
+                                            "prompts",
+                                            "prompt",
+                                            "PromptTemplate",
+                                        ],
+                                        "kwargs": {
+                                            "template": "{input}",
+                                            "input_variables": ["input"],
+                                            "template_format": "f-string",
+                                            "partial_variables": {},
+                                        },
+                                    },
+                                ]
+                            },
+                        },
+                    ],
+                    "input_variables": ["img", "input"],
+                },
+            }
+        )
+    )

libs/langchain/tests/unit_tests/load/test_serializable.py

@@ -40,8 +40,21 @@ def import_all_modules(package_name: str) -> dict:
 
 
 def test_serializable_mapping() -> None:
+    # This should have had a different namespace, as it was never
+    # exported from the langchain module, but we keep for whoever has
+    # already serialized it.
+    to_skip = {
+        ("langchain", "prompts", "image", "ImagePromptTemplate"): (
+            "langchain_core",
+            "prompts",
+            "image",
+            "ImagePromptTemplate",
+        ),
+    }
     serializable_modules = import_all_modules("langchain")
-    missing = set(SERIALIZABLE_MAPPING).difference(serializable_modules)
+    missing = set(SERIALIZABLE_MAPPING).difference(
+        set(serializable_modules).union(to_skip)
+    )
     assert missing == set()
     extra = set(serializable_modules).difference(SERIALIZABLE_MAPPING)
     assert extra == set()