# BaseQAWithSourcesChain

> **Class** in `langchain_classic`

📖 [View in docs](https://reference.langchain.com/python/langchain-classic/chains/qa_with_sources/base/BaseQAWithSourcesChain)

Question answering chain with sources over documents.

## Signature

```python
BaseQAWithSourcesChain()
```

## Extends

- `Chain`
- `ABC`

## Properties

- `combine_documents_chain`
- `question_key`
- `input_docs_key`
- `answer_key`
- `sources_answer_key`
- `return_source_documents`
- `model_config`
- `input_keys`
- `output_keys`

## Methods

- [`from_llm()`](https://reference.langchain.com/python/langchain-classic/chains/qa_with_sources/base/BaseQAWithSourcesChain/from_llm)
- [`from_chain_type()`](https://reference.langchain.com/python/langchain-classic/chains/qa_with_sources/base/BaseQAWithSourcesChain/from_chain_type)
- [`validate_naming()`](https://reference.langchain.com/python/langchain-classic/chains/qa_with_sources/base/BaseQAWithSourcesChain/validate_naming)

## ⚠️ Deprecated

Deprecated since version 0.2.13.

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[View source on GitHub](https://github.com/langchain-ai/langchain/blob/f0c5a28fa05adcda89aebcb449d897245ab21fa4/libs/langchain/langchain_classic/chains/qa_with_sources/base.py#L37)