fix(core): better error message for empty var names (#32073)

Previously, we hit an index out of range error with empty variable names
(accessing tag[0]), now we through a slightly nicer error

---------

Co-authored-by: Mason Daugherty <mason@langchain.dev>
This commit is contained in:
Isaac Francisco 2025-07-18 14:00:02 -07:00 committed by GitHub
parent 427d2d6397
commit 98bfd57a76
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2 changed files with 12 additions and 0 deletions

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@ -150,6 +150,11 @@ def parse_tag(template: str, l_del: str, r_del: str) -> tuple[tuple[str, str], s
msg = f"unclosed tag at line {_CURRENT_LINE}"
raise ChevronError(msg) from e
# Check for empty tags
if not tag.strip():
msg = f"empty tag at line {_CURRENT_LINE}"
raise ChevronError(msg)
# Find the type meaning of the first character
tag_type = tag_types.get(tag[0], "variable")

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@ -2,6 +2,7 @@ from functools import partial
from inspect import isclass
from typing import Any, Union, cast
import pytest
from pydantic import BaseModel
from langchain_core.language_models import FakeListChatModel
@ -10,6 +11,7 @@ from langchain_core.load.load import loads
from langchain_core.messages import HumanMessage
from langchain_core.prompts.structured import StructuredPrompt
from langchain_core.runnables.base import Runnable, RunnableLambda
from langchain_core.utils.mustache import ChevronError
from langchain_core.utils.pydantic import is_basemodel_subclass
@ -128,3 +130,8 @@ def test_structured_prompt_template_format() -> None:
assert prompt.invoke({"person": {"name": "foo"}}).to_messages() == [
HumanMessage("hi foo")
]
def test_structured_prompt_template_empty_vars() -> None:
with pytest.raises(ChevronError, match="empty tag"):
StructuredPrompt([("human", "hi {{}}")], schema={}, template_format="mustache")