core[patch]: fix chat prompt partial messages placeholder var (#16918)

libs/core/langchain_core/prompts/base.py

@@ -47,9 +47,7 @@ class BasePromptTemplate(
     If not provided, all variables are assumed to be strings."""
     output_parser: Optional[BaseOutputParser] = None
     """How to parse the output of calling an LLM on this formatted prompt."""
-    partial_variables: Mapping[str, Union[str, Callable[[], str]]] = Field(
-        default_factory=dict
-    )
+    partial_variables: Mapping[str, Any] = Field(default_factory=dict)
 
     @classmethod
     def get_lc_namespace(cls) -> List[str]:
@@ -143,8 +141,7 @@ class BasePromptTemplate(
     def _merge_partial_and_user_variables(self, **kwargs: Any) -> Dict[str, Any]:
         # Get partial params:
         partial_kwargs = {
-            k: v if isinstance(v, str) else v()
-            for k, v in self.partial_variables.items()
+            k: v if not callable(v) else v() for k, v in self.partial_variables.items()
         }
         return {**partial_kwargs, **kwargs}
 

libs/core/langchain_core/prompts/chat.py

@@ -5,7 +5,6 @@ from abc import ABC, abstractmethod
 from pathlib import Path
 from typing import (
     Any,
-    Callable,
     Dict,
     List,
     Optional,
@@ -130,13 +129,7 @@ class MessagesPlaceholder(BaseMessagePromptTemplate):
                 f"variable {self.variable_name} should be a list of base messages, "
                 f"got {value}"
             )
-        for v in convert_to_messages(value):
-            if not isinstance(v, BaseMessage):
-                raise ValueError(
-                    f"variable {self.variable_name} should be a list of base messages,"
-                    f" got {value}"
-                )
-        return value
+        return convert_to_messages(value)
 
     @property
     def input_variables(self) -> List[str]:
@@ -755,13 +748,20 @@ class ChatPromptTemplate(BaseChatPromptTemplate):
 
         # Automatically infer input variables from messages
         input_vars: Set[str] = set()
+        partial_vars: Dict[str, Any] = {}
         for _message in _messages:
-            if isinstance(
+            if isinstance(_message, MessagesPlaceholder) and _message.optional:
+                partial_vars[_message.variable_name] = []
+            elif isinstance(
                 _message, (BaseChatPromptTemplate, BaseMessagePromptTemplate)
             ):
                 input_vars.update(_message.input_variables)
 
-        return cls(input_variables=sorted(input_vars), messages=_messages)
+        return cls(
+            input_variables=sorted(input_vars),
+            messages=_messages,
+            partial_variables=partial_vars,
+        )
 
     def format(self, **kwargs: Any) -> str:
         """Format the chat template into a string.
@@ -799,7 +799,7 @@ class ChatPromptTemplate(BaseChatPromptTemplate):
                 raise ValueError(f"Unexpected input: {message_template}")
         return result
 
-    def partial(self, **kwargs: Union[str, Callable[[], str]]) -> ChatPromptTemplate:
+    def partial(self, **kwargs: Any) -> ChatPromptTemplate:
         """Get a new ChatPromptTemplate with some input variables already filled in.
 
         Args:

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

@@ -503,9 +503,27 @@ def test_messages_placeholder() -> None:
         prompt.format_messages()
     prompt = MessagesPlaceholder("history", optional=True)
     assert prompt.format_messages() == []
-    prompt.format_messages(
+    assert prompt.format_messages(
         history=[("system", "You are an AI assistant."), "Hello!"]
     ) == [
         SystemMessage(content="You are an AI assistant."),
         HumanMessage(content="Hello!"),
     ]
+
+
+def test_chat_prompt_message_placeholder_partial() -> None:
+    prompt = ChatPromptTemplate.from_messages([MessagesPlaceholder("history")])
+    prompt = prompt.partial(history=[("system", "foo")])
+    assert prompt.format_messages() == [SystemMessage(content="foo")]
+    assert prompt.format_messages(history=[("system", "bar")]) == [
+        SystemMessage(content="bar")
+    ]
+
+    prompt = ChatPromptTemplate.from_messages(
+        [
+            MessagesPlaceholder("history", optional=True),
+        ]
+    )
+    assert prompt.format_messages() == []
+    prompt = prompt.partial(history=[("system", "foo")])
+    assert prompt.format_messages() == [SystemMessage(content="foo")]

libs/core/tests/unit_tests/runnables/__snapshots__/test_runnable.ambr

@@ -1544,7 +1544,8 @@
                           }
                         }
                       }
-                    ]
+                    ],
+                    "partial_variables": {}
                   }
                 },
                 "middle": [],
@@ -1617,7 +1618,8 @@
                           }
                         }
                       }
-                    ]
+                    ],
+                    "partial_variables": {}
                   }
                 },
                 "middle": [],

libs/langchain/tests/unit_tests/load/__snapshots__/test_dump.ambr

@@ -191,7 +191,8 @@
                 }
               }
             }
-          ]
+          ],
+          "partial_variables": {}
         }
       }
     }