[Feat] Accept non-dict if only 1 prompt input variable

libs/core/langchain_core/prompts/base.py

@@ -89,10 +89,15 @@ def get_input_schema(
 
     def _format_prompt_with_error_handling(self, inner_input: Dict) -> PromptValue:
         if not isinstance(inner_input, dict):
-            raise TypeError(
-                f"Expected mapping type as input to {self.__class__.__name__}. "
-                f"Received {type(inner_input)}."
-            )
+            if len(self.input_variables) == 1:
+                var_name = self.input_variables[0]
+                inner_input = {var_name: inner_input}
+
+            else:
+                raise TypeError(
+                    f"Expected mapping type as input to {self.__class__.__name__}. "
+                    f"Received {type(inner_input)}."
+                )
         missing = set(self.input_variables).difference(inner_input)
         if missing:
             raise KeyError(

libs/core/langchain_core/prompts/chat.py

@@ -574,11 +574,50 @@ class ChatPromptTemplate(BaseChatPromptTemplate):
                 ("human", "{user_input}"),
             ])
 
-            messages = template.format_messages(
-                name="Bob",
-                user_input="What is your name?"
+            prompt_value = template.invoke(
+                {
+                    "name": "Bob",
+                    "user_input": "What is your name?"
+                }
             )
-    """
+            # Output:
+            # ChatPromptValue(
+            #    messages=[
+            #        SystemMessage(content='You are a helpful AI bot. Your name is Bob.'),
+            #        HumanMessage(content='Hello, how are you doing?'),
+            #        AIMessage(content="I'm doing well, thanks!"),
+            #        HumanMessage(content='What is your name?')
+            #    ]
+            #)
+
+        If your prompt has only a single input variable (i.e., 1 instance of "{variable_nams}"),
+        and you invoke the template with a non-dict object, the prompt template will
+        inject the provided argument into that variable location.
+
+        Single-variable template:
+
+        .. code-block:: python
+
+            from langchain_core.prompts import ChatPromptTemplate
+
+            template = ChatPromptTemplate.from_messages([
+                ("system", "You are a helpful AI bot. Your name is Carl."),
+                ("human", "{user_input}"),
+            ])
+
+            prompt_value = template.invoke("Hello, there!")
+            # Equivalent to
+            # prompt_value = template.invoke({"user_input": "Hello, there!"})
+
+            # Output:
+            #  ChatPromptValue(
+            #     messages=[
+            #         SystemMessage(content='You are a helpful AI bot. Your name is Carl.'),
+            #         HumanMessage(content='Hello, there!'),
+            #     ]
+            # )
+
+    """  # noqa: E501
 
     input_variables: List[str]
     """List of input variables in template messages. Used for validation."""

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

@@ -533,3 +533,16 @@ def test_chat_prompt_message_placeholder_partial() -> None:
     assert prompt.format_messages() == []
     prompt = prompt.partial(history=[("system", "foo")])
     assert prompt.format_messages() == [SystemMessage(content="foo")]
+
+
+def test_messages_prompt_accepts_list() -> None:
+    prompt = ChatPromptTemplate.from_messages([MessagesPlaceholder("history")])
+    value = prompt.invoke([("user", "Hi there")])  # type: ignore
+    assert value.to_messages() == [HumanMessage(content="Hi there")]
+
+    # Assert still raises a nice error
+    prompt = ChatPromptTemplate.from_messages(
+        [("system", "You are a {foo}"), MessagesPlaceholder("history")]
+    )
+    with pytest.raises(TypeError):
+        prompt.invoke([("user", "Hi there")])  # type: ignore