core(patch):fix partial_variables not working with SystemMessagePromptTemplate (#20711)

- **Issue:**  close #17560
- @baskaryan, @eyurtsev

libs/core/langchain_core/prompts/chat.py

@@ -407,6 +407,8 @@ class _StringImageMessagePromptTemplate(BaseMessagePromptTemplate):
         cls: Type[_StringImageMessagePromptTemplateT],
         template: Union[str, List[Union[str, _TextTemplateParam, _ImageTemplateParam]]],
         template_format: str = "f-string",
+        *,
+        partial_variables: Optional[Dict[str, Any]] = None,
         **kwargs: Any,
     ) -> _StringImageMessagePromptTemplateT:
         """Create a class from a string template.
@@ -414,6 +416,7 @@ class _StringImageMessagePromptTemplate(BaseMessagePromptTemplate):
         Args:
             template: a template.
             template_format: format of the template.
+            partial_variables: A dictionary of variables that can be used too partially.
             **kwargs: keyword arguments to pass to the constructor.
 
         Returns:
@@ -421,10 +424,16 @@ class _StringImageMessagePromptTemplate(BaseMessagePromptTemplate):
         """
         if isinstance(template, str):
             prompt: Union[StringPromptTemplate, List] = PromptTemplate.from_template(
-                template, template_format=template_format
+                template,
+                template_format=template_format,
+                partial_variables=partial_variables,
             )
             return cls(prompt=prompt, **kwargs)
         elif isinstance(template, list):
+            if (partial_variables is not None) and len(partial_variables) > 0:
+                raise ValueError(
+                    "Partial variables are not supported for list of templates."
+                )
             prompt = []
             for tmpl in template:
                 if isinstance(tmpl, str) or isinstance(tmpl, dict) and "text" in tmpl:

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

@@ -99,6 +99,79 @@ def test_create_chat_prompt_template_from_template_partial() -> None:
     assert output_prompt.prompt == expected_prompt
 
 
+def test_create_system_message_prompt_template_from_template_partial() -> None:
+    """Create a system message prompt template with partials."""
+
+    graph_creator_content = """
+    Your instructions are:
+    {instructions}
+    History:
+    {history}
+    """
+    json_prompt_instructions: dict = {}
+    graph_analyst_template = SystemMessagePromptTemplate.from_template(
+        template=graph_creator_content,
+        input_variables=["history"],
+        partial_variables={"instructions": json_prompt_instructions},
+    )
+    assert graph_analyst_template.format(history="history") == SystemMessage(
+        content="\n    Your instructions are:\n  "
+        "  {}\n    History:\n    "
+        "history\n    "
+    )
+
+
+def test_create_system_message_prompt_list_template() -> None:
+    graph_creator_content1 = """
+    This is the prompt for the first test:
+    {variables}
+    """
+    graph_creator_content2 = """
+    This is the prompt for the second test:
+        {variables}
+        """
+    graph_analyst_template = SystemMessagePromptTemplate.from_template(
+        template=[graph_creator_content1, graph_creator_content2],
+        input_variables=["variables"],
+    )
+    assert graph_analyst_template.format(variables="foo") == SystemMessage(
+        content=[
+            {
+                "type": "text",
+                "text": "\n    This is the prompt for the first test:\n    foo\n    ",
+            },
+            {
+                "type": "text",
+                "text": "\n    This is the prompt for "
+                "the second test:\n        foo\n        ",
+            },
+        ]
+    )
+
+
+def test_create_system_message_prompt_list_template_partial_variables_not_null() -> (
+    None
+):
+    graph_creator_content1 = """
+    This is the prompt for the first test:
+    {variables}
+    """
+    graph_creator_content2 = """
+    This is the prompt for the second test:
+        {variables}
+        """
+
+    try:
+        graph_analyst_template = SystemMessagePromptTemplate.from_template(
+            template=[graph_creator_content1, graph_creator_content2],
+            input_variables=["variables"],
+            partial_variables={"variables": "foo"},
+        )
+        graph_analyst_template.format(variables="foo")
+    except ValueError as e:
+        assert str(e) == "Partial variables are not supported for list of templates."
+
+
 def test_message_prompt_template_from_template_file() -> None:
     expected = ChatMessagePromptTemplate(
         prompt=PromptTemplate(