PydanticUserError when instantiating SyntheticDataGenerator

[zakhikhan]

I am attempting to use this library to generate tabular synthetic data. Any time I try to instantiate the SyntheticDataGenerator class from tabular_synthetic_data (either through the create_openai_data_generator function or directly), I get an error like the below:

PydanticUserError: SyntheticDataGenerator is not fully defined; you should define BaseCache, then call SyntheticDataGenerator.model_rebuild().

For further information visit https://errors.pydantic.dev/2.10/u/class-not-fully-defined

versions (all installed via pip)
langchain_experimental: 0.3.3
langchain: 0.3.11
langchain_openai 0.2.12
langchain_core: 0.3.24

Here is simplified example code that causes the error:

from langchain_openai import ChatOpenAI
from langchain_core.prompts import FewShotPromptTemplate
from langchain_experimental.tabular_synthetic_data.base import SyntheticDataGenerator
from pydantic import BaseModel

OPENAI_TEMPLATE = PromptTemplate(input_variables=["example"], template="{example}")

examples = [
    {"example": "toothbrush_name: Crest Brush, price: 10"},
    {"example": "toothbrush_name: Generic toothbrush, price: 5"}
]

class ToothbrushSchema(BaseModel):
    toothbrush_name: str
    price: int
    
output_schema=ToothbrushSchema

llm=ChatOpenAI(temperature=1)

prompt_template = FewShotPromptTemplate(
    examples=examples,
    suffix=("Generate synthetic data about {subject}."),
    input_variables=["subject"],
    example_prompt=OPENAI_TEMPLATE,
)

prompt=prompt_template

chain = llm.with_structured_output(ToothbrushSchema)

sdg = SyntheticDataGenerator(template=prompt_template,llm_chain=chain)

[zakhikhan]

I got it to work by creating a dev install of langchain_experimental and editing line 27 of tabular_synthetic_data/base.py in the SyntheticDataGenerator class

BEFORE:
llm: Optional[BaseLanguageModel] = None

AFTER:
llm: Optional[BaseChatModel] = None

I am not sure why this worked. BaseLanguageModel is the parent class of BaseChatModel.