deepset-ai--haystack/docs-website/versioned_docs/version-2.21/pipeline-components/rankers/pyversityranker.mdx

---
title: "PyversityRanker"
id: pyversityranker
slug: "/pyversityranker"
description: "Use this component to rerank documents by balancing relevance and diversity using pyversity's diversification algorithms."
---

# PyversityRanker

Use this component to rerank documents by balancing relevance and diversity using pyversity's diversification algorithms.

<div className="key-value-table">

|  |  |
| --- | --- |
| **Most common position in a pipeline** | In a query pipeline, after a dense [Retriever](../retrievers.mdx) with `return_embedding=True` |
| **Mandatory init variables** | None |
| **Mandatory run variables** | `documents`: A list of document objects, each with `score` and `embedding` set |
| **Output variables** | `documents`: A list of document objects |
| **API reference** | [Pyversity](/reference/integrations-pyversity) |
| **GitHub link** | https://github.com/deepset-ai/haystack-core-integrations/tree/main/integrations/pyversity |

</div>

## Overview

`PyversityRanker` reranks `Documents` using [pyversity](https://github.com/Pringled/pyversity)'s diversification algorithms. Unlike similarity-based rankers, it balances **relevance and diversity** - so the output isn't just the most relevant documents, but a varied selection that avoids redundancy.

Documents must have both `score` and `embedding` populated. This makes it a natural fit after a dense retriever such as `InMemoryEmbeddingRetriever` configured with `return_embedding=True`. Documents missing either field are skipped with a warning.

The key parameters are:

- `strategy`: The diversification algorithm to use. Defaults to `Strategy.DPP` (Determinantal Point Process). `Strategy.MMR` (Maximal Marginal Relevance) is another popular option.
- `diversity`: A float in `[0, 1]` controlling the relevance–diversity trade-off. `0.0` keeps the most relevant documents; `1.0` maximises diversity regardless of relevance. Defaults to `0.5`.
- `top_k`: The number of documents to return. If `None`, all documents are returned in diversified order.

### Installation

To start using this integration with Haystack, install the package with:

```shell
pip install pyversity-haystack
```

## Usage

### On its own

This example uses `PyversityRanker` to rerank five documents. Each document must have a `score` and `embedding` set. The ranker returns the top 3 documents using the MMR strategy with a diversity of `0.7`.

```python
from haystack import Document
from pyversity import Strategy

from haystack_integrations.components.rankers.pyversity import PyversityRanker

documents = [
    Document(
        content="Paris is the capital of France.",
        score=0.95,
        embedding=[0.9, 0.1, 0.0, 0.0],
    ),
    Document(
        content="The Eiffel Tower is located in Paris.",
        score=0.90,
        embedding=[0.8, 0.2, 0.0, 0.0],
    ),
    Document(
        content="Berlin is the capital of Germany.",
        score=0.85,
        embedding=[0.0, 0.0, 0.9, 0.1],
    ),
    Document(
        content="The Brandenburg Gate is in Berlin.",
        score=0.80,
        embedding=[0.0, 0.0, 0.8, 0.2],
    ),
    Document(
        content="France borders Spain to the south.",
        score=0.75,
        embedding=[0.5, 0.5, 0.0, 0.0],
    ),
]

ranker = PyversityRanker(top_k=3, strategy=Strategy.MMR, diversity=0.7)
result = ranker.run(documents=documents)

for doc in result["documents"]:
    print(f"{doc.score:.2f}  {doc.content}")
```

### In a pipeline

Below is an example of a pipeline that embeds documents and stores them in an `InMemoryDocumentStore`. It then retrieves the top 6 documents using `InMemoryEmbeddingRetriever` and reranks them with `PyversityRanker` to return 3 diverse results.

Note that the retriever must be configured with `return_embedding=True` so that documents have embeddings available for the ranker.

```python
from haystack import Document, Pipeline
from haystack.components.embedders import (
    SentenceTransformersDocumentEmbedder,
    SentenceTransformersTextEmbedder,
)
from haystack.components.retrievers import InMemoryEmbeddingRetriever
from haystack.document_stores.in_memory import InMemoryDocumentStore
from pyversity import Strategy

from haystack_integrations.components.rankers.pyversity import PyversityRanker

# Index documents
document_store = InMemoryDocumentStore()

raw_documents = [
    Document(content="Paris is the capital of France."),
    Document(content="The Eiffel Tower is located in Paris."),
    Document(content="Berlin is the capital of Germany."),
    Document(content="The Brandenburg Gate is in Berlin."),
    Document(content="France borders Spain to the south."),
    Document(content="The Louvre is the world's largest art museum and is in Paris."),
    Document(content="Munich is the capital of Bavaria."),
    Document(content="The Rhine river flows through Germany and France."),
]

doc_embedder = SentenceTransformersDocumentEmbedder()
documents_with_embeddings = doc_embedder.run(raw_documents)["documents"]
document_store.write_documents(documents_with_embeddings)

# Build pipeline
pipeline = Pipeline()
pipeline.add_component("text_embedder", SentenceTransformersTextEmbedder())
pipeline.add_component(
    "retriever",
    InMemoryEmbeddingRetriever(
        document_store=document_store,
        top_k=6,
        return_embedding=True,
    ),
)
pipeline.add_component(
    "ranker",
    PyversityRanker(top_k=3, strategy=Strategy.MMR, diversity=0.7),
)

pipeline.connect("text_embedder.embedding", "retriever.query_embedding")
pipeline.connect("retriever.documents", "ranker.documents")

# Run
result = pipeline.run(
    {"text_embedder": {"text": "What are the famous landmarks in France?"}},
)

for doc in result["ranker"]["documents"]:
    print(f"{doc.score:.4f}  {doc.content}")
```

:::note[Embeddings required]

`PyversityRanker` requires documents to have both `score` and `embedding` set. When using a dense retriever, make sure to pass `return_embedding=True`. Documents missing either field are skipped with a warning.

:::