core: allow passing message dicts into ChatPromptTemplate (#29363)

Co-authored-by: Erick Friis <erick@langchain.dev>

libs/core/langchain_core/prompts/chat.py

@@ -828,6 +828,7 @@ MessageLikeRepresentation = Union[
         Union[str, list[dict], list[object]],
     ],
     str,
+    dict,
 ]
 
 
@@ -1461,7 +1462,15 @@ def _convert_to_message(
         _message = _create_template_from_message_type(
             "human", message, template_format=template_format
         )
-    elif isinstance(message, tuple):
+    elif isinstance(message, (tuple, dict)):
+        if isinstance(message, dict):
+            if set(message.keys()) != {"content", "role"}:
+                msg = (
+                    "Expected dict to have exact keys 'role' and 'content'."
+                    f" Got: {message}"
+                )
+                raise ValueError(msg)
+            message = (message["role"], message["content"])
         if len(message) != 2:
             msg = f"Expected 2-tuple of (role, template), got {message}"
             raise ValueError(msg)

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

@@ -824,6 +824,41 @@ def test_chat_prompt_message_placeholder_tuple() -> None:
         assert optional_prompt.format_messages() == []
 
 
+def test_chat_prompt_message_placeholder_dict() -> None:
+    prompt = ChatPromptTemplate([{"role": "placeholder", "content": "{convo}"}])
+    assert prompt.format_messages(convo=[("user", "foo")]) == [
+        HumanMessage(content="foo")
+    ]
+
+    assert prompt.format_messages() == []
+
+    # Is optional = True
+    optional_prompt = ChatPromptTemplate(
+        [{"role": "placeholder", "content": ["{convo}", False]}]
+    )
+    assert optional_prompt.format_messages(convo=[("user", "foo")]) == [
+        HumanMessage(content="foo")
+    ]
+    with pytest.raises(KeyError):
+        assert optional_prompt.format_messages() == []
+
+
+def test_chat_prompt_message_dict() -> None:
+    prompt = ChatPromptTemplate(
+        [{"role": "system", "content": "foo"}, {"role": "user", "content": "bar"}]
+    )
+    assert prompt.format_messages() == [
+        SystemMessage(content="foo"),
+        HumanMessage(content="bar"),
+    ]
+
+    with pytest.raises(ValueError):
+        ChatPromptTemplate([{"role": "system", "content": False}])
+
+    with pytest.raises(ValueError):
+        ChatPromptTemplate([{"role": "foo", "content": "foo"}])
+
+
 async def test_messages_prompt_accepts_list() -> None:
     prompt = ChatPromptTemplate([MessagesPlaceholder("history")])
     value = prompt.invoke([("user", "Hi there")])  # type: ignore