allow passing of messages into prompt template (#1505)

langchain/prompts/chat.py

@@ -120,7 +120,7 @@ class ChatPromptValue(PromptValue):
 
 class ChatPromptTemplate(BasePromptTemplate, ABC):
     input_variables: List[str]
-    messages: List[BaseMessagePromptTemplate]
+    messages: List[Union[BaseMessagePromptTemplate, BaseMessage]]
 
     @classmethod
     def from_role_strings(
@@ -146,11 +146,12 @@ class ChatPromptTemplate(BasePromptTemplate, ABC):
 
     @classmethod
     def from_messages(
-        cls, messages: Sequence[BaseMessagePromptTemplate]
+        cls, messages: Sequence[Union[BaseMessagePromptTemplate, BaseMessage]]
     ) -> ChatPromptTemplate:
         input_vars = set()
         for message in messages:
-            input_vars.update(message.input_variables)
+            if isinstance(message, BaseMessagePromptTemplate):
+                input_vars.update(message.input_variables)
         return cls(input_variables=list(input_vars), messages=messages)
 
     def format(self, **kwargs: Any) -> str:
@@ -159,11 +160,18 @@ class ChatPromptTemplate(BasePromptTemplate, ABC):
     def format_prompt(self, **kwargs: Any) -> PromptValue:
         result = []
         for message_template in self.messages:
-            rel_params = {
-                k: v for k, v in kwargs.items() if k in message_template.input_variables
-            }
-            message = message_template.format_messages(**rel_params)
-            result.extend(message)
+            if isinstance(message_template, BaseMessage):
+                result.extend([message_template])
+            elif isinstance(message_template, BaseMessagePromptTemplate):
+                rel_params = {
+                    k: v
+                    for k, v in kwargs.items()
+                    if k in message_template.input_variables
+                }
+                message = message_template.format_messages(**rel_params)
+                result.extend(message)
+            else:
+                raise ValueError(f"Unexpected input: {message_template}")
         return ChatPromptValue(messages=result)
 
     def partial(self, **kwargs: Union[str, Callable[[], str]]) -> BasePromptTemplate:

tests/unit_tests/prompts/test_chat.py

@@ -10,6 +10,7 @@ from langchain.prompts.chat import (
     HumanMessagePromptTemplate,
     SystemMessagePromptTemplate,
 )
+from langchain.schema import HumanMessage
 
 
 def create_messages() -> List[BaseMessagePromptTemplate]:
@@ -89,3 +90,17 @@ def test_chat_prompt_template_from_messages() -> None:
         ["context", "foo", "bar"]
     )
     assert len(chat_prompt_template.messages) == 4
+
+
+def test_chat_prompt_template_with_messages() -> None:
+    messages = create_messages() + [HumanMessage(content="foo")]
+    chat_prompt_template = ChatPromptTemplate.from_messages(messages)
+    assert sorted(chat_prompt_template.input_variables) == sorted(
+        ["context", "foo", "bar"]
+    )
+    assert len(chat_prompt_template.messages) == 5
+    prompt_value = chat_prompt_template.format_prompt(
+        context="see", foo="this", bar="magic"
+    )
+    prompt_value_messages = prompt_value.to_messages()
+    assert prompt_value_messages[-1] == HumanMessage(content="foo")