core[patch]: Fix regression requiring input_variables in few chat prompt templates (#24360)

* Fix regression that requires users passing input_variables=[].

* Regression introduced by my own changes to this PR:
https://github.com/langchain-ai/langchain/pull/22851

libs/core/langchain_core/prompts/few_shot.py

@@ -18,7 +18,7 @@ from langchain_core.prompts.string import (
     check_valid_template,
     get_template_variables,
 )
-from langchain_core.pydantic_v1 import BaseModel, Extra, root_validator
+from langchain_core.pydantic_v1 import BaseModel, Extra, Field, root_validator
 
 
 class _FewShotPromptTemplateMixin(BaseModel):
@@ -135,6 +135,12 @@ class FewShotPromptTemplate(_FewShotPromptTemplateMixin, StringPromptTemplate):
     template_format: Literal["f-string", "jinja2"] = "f-string"
     """The format of the prompt template. Options are: 'f-string', 'jinja2'."""
 
+    def __init__(self, **kwargs: Any) -> None:
+        """Initialize the few shot prompt template."""
+        if "input_variables" not in kwargs and "example_prompt" in kwargs:
+            kwargs["input_variables"] = kwargs["example_prompt"].input_variables
+        super().__init__(**kwargs)
+
     @root_validator(pre=False, skip_on_failure=True)
     def template_is_valid(cls, values: Dict) -> Dict:
         """Check that prefix, suffix, and input variables are consistent."""
@@ -351,14 +357,18 @@ class FewShotChatMessagePromptTemplate(
             chain.invoke({"input": "What's 3+3?"})
     """
 
+    input_variables: List[str] = Field(default_factory=list)
+    """A list of the names of the variables the prompt template will use
+    to pass to the example_selector, if provided."""
+
+    example_prompt: Union[BaseMessagePromptTemplate, BaseChatPromptTemplate]
+    """The class to format each example."""
+
     @classmethod
     def is_lc_serializable(cls) -> bool:
         """Return whether or not the class is serializable."""
         return False
 
-    example_prompt: Union[BaseMessagePromptTemplate, BaseChatPromptTemplate]
-    """The class to format each example."""
-
     class Config:
         """Configuration for this pydantic object."""
 

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

@@ -58,6 +58,17 @@ def test_suffix_only() -> None:
     assert output == expected_output
 
 
+def test_auto_infer_input_variables() -> None:
+    """Test prompt works with just a suffix."""
+    suffix = "This is a {foo} test."
+    prompt = FewShotPromptTemplate(
+        suffix=suffix,
+        examples=[],
+        example_prompt=EXAMPLE_PROMPT,
+    )
+    assert prompt.input_variables == ["foo"]
+
+
 def test_prompt_missing_input_variables() -> None:
     """Test error is raised when input variables are not provided."""
     # Test when missing in suffix
@@ -422,6 +433,30 @@ def test_few_shot_chat_message_prompt_template_with_selector() -> None:
     assert messages == expected
 
 
+def test_few_shot_chat_message_prompt_template_infer_input_variables() -> None:
+    """Check that it can infer input variables if not provided."""
+    examples = [
+        {"input": "2+2", "output": "4"},
+        {"input": "2+3", "output": "5"},
+    ]
+    example_selector = AsIsSelector(examples)
+    example_prompt = ChatPromptTemplate.from_messages(
+        [
+            HumanMessagePromptTemplate.from_template("{input}"),
+            AIMessagePromptTemplate.from_template("{output}"),
+        ]
+    )
+
+    few_shot_prompt = FewShotChatMessagePromptTemplate(
+        example_prompt=example_prompt,
+        example_selector=example_selector,
+    )
+
+    # The prompt template does not have any inputs! They
+    # have already been filled in.
+    assert few_shot_prompt.input_variables == []
+
+
 class AsyncAsIsSelector(BaseExampleSelector):
     """An example selector for testing purposes.