LlamaIndex is the leading data framework for building LLM applications

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Installation and Setup

You need to install the llama-index python package. 
pip install llama-index
See the installation instructions.

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Retrievers

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LlamaIndexRetriever

It is used for the question-answering with sources over an LlamaIndex data structure. 
from langchain_community.retrievers.llama_index import LlamaIndexRetriever

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LlamaIndexGraphRetriever

It is used for question-answering with sources over an LlamaIndex graph data structure. 
from langchain_community.retrievers.llama_index import LlamaIndexGraphRetriever