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# Source code for langchain.chains.combine_documents.reduce

    """Combine many documents together by recursively reducing them."""

    from __future__ import annotations

    from typing import Any, Callable, List, Optional, Protocol, Tuple

    from langchain_core.callbacks import Callbacks
    from langchain_core.documents import Document
    from langchain_core.pydantic_v1 import Extra

    from langchain.chains.combine_documents.base import BaseCombineDocumentsChain

    class CombineDocsProtocol(Protocol):
        """Interface for the combine_docs method."""

        def __call__(self, docs: List[Document], **kwargs: Any) -> str:
            """Interface for the combine_docs method."""

    class AsyncCombineDocsProtocol(Protocol):
        """Interface for the combine_docs method."""

        async def __call__(self, docs: List[Document], **kwargs: Any) -> str:
            """Async interface for the combine_docs method."""

    def split_list_of_docs(
        docs: List[Document], length_func: Callable, token_max: int, **kwargs: Any
    ) -> List[List[Document]]:
        """Split Documents into subsets that each meet a cumulative length constraint.

        Args:
            docs: The full list of Documents.
            length_func: Function for computing the cumulative length of a set of Documents.
            token_max: The maximum cumulative length of any subset of Documents.
            **kwargs: Arbitrary additional keyword params to pass to each call of the
                length_func.

        Returns:
            A List[List[Document]].
        """
        new_result_doc_list = []
        _sub_result_docs = []
        for doc in docs:
            _sub_result_docs.append(doc)
            _num_tokens = length_func(_sub_result_docs, **kwargs)
            if _num_tokens > token_max:
                if len(_sub_result_docs) == 1:
                    raise ValueError(
                        "A single document was longer than the context length,"
                        " we cannot handle this."
                    )
                new_result_doc_list.append(_sub_result_docs[:-1])
                _sub_result_docs = _sub_result_docs[-1:]
        new_result_doc_list.append(_sub_result_docs)
        return new_result_doc_list

    def collapse_docs(
        docs: List[Document],
        combine_document_func: CombineDocsProtocol,
        **kwargs: Any,
    ) -> Document:
        """Execute a collapse function on a set of documents and merge their metadatas.

        Args:
            docs: A list of Documents to combine.
            combine_document_func: A function that takes in a list of Documents and
                optionally addition keyword parameters and combines them into a single
                string.
            **kwargs: Arbitrary additional keyword params to pass to the
                combine_document_func.

        Returns:
            A single Document with the output of combine_document_func for the page content
                and the combined metadata's of all the input documents. All metadata values
                are strings, and where there are overlapping keys across documents the
                values are joined by ", ".
        """
        result = combine_document_func(docs, **kwargs)
        combined_metadata = {k: str(v) for k, v in docs[0].metadata.items()}
        for doc in docs[1:]:
            for k, v in doc.metadata.items():
                if k in combined_metadata:
                    combined_metadata[k] += f", {v}"
                else:
                    combined_metadata[k] = str(v)
        return Document(page_content=result, metadata=combined_metadata)

    async def acollapse_docs(
        docs: List[Document],
        combine_document_func: AsyncCombineDocsProtocol,
        **kwargs: Any,
    ) -> Document:
        """Execute a collapse function on a set of documents and merge their metadatas.

        Args:
            docs: A list of Documents to combine.
            combine_document_func: A function that takes in a list of Documents and
                optionally addition keyword parameters and combines them into a single
                string.
            **kwargs: Arbitrary additional keyword params to pass to the
                combine_document_func.

        Returns:
            A single Document with the output of combine_document_func for the page content
                and the combined metadata's of all the input documents. All metadata values
                are strings, and where there are overlapping keys across documents the
                values are joined by ", ".
        """
        result = await combine_document_func(docs, **kwargs)
        combined_metadata = {k: str(v) for k, v in docs[0].metadata.items()}
        for doc in docs[1:]:
            for k, v in doc.metadata.items():
                if k in combined_metadata:
                    combined_metadata[k] += f", {v}"
                else:
                    combined_metadata[k] = str(v)
        return Document(page_content=result, metadata=combined_metadata)

    class ReduceDocumentsChain(BaseCombineDocumentsChain):
        """Combine documents by recursively reducing them.

        This involves

        - combine_documents_chain

        - collapse_documents_chain

        `combine_documents_chain` is ALWAYS provided. This is final chain that is called.
        We pass all previous results to this chain, and the output of this chain is
        returned as a final result.

        `collapse_documents_chain` is used if the documents passed in are too many to all
        be passed to `combine_documents_chain` in one go. In this case,
        `collapse_documents_chain` is called recursively on as big of groups of documents
        as are allowed.

        Example:
            .. code-block:: python

                from langchain.chains import (
                    StuffDocumentsChain, LLMChain, ReduceDocumentsChain
                )
                from langchain_core.prompts import PromptTemplate
                from langchain_community.llms import OpenAI

                # This controls how each document will be formatted. Specifically,
                # it will be passed to `format_document` - see that function for more
                # details.
                document_prompt = PromptTemplate(
                    input_variables=["page_content"],
                     template="{page_content}"
                )
                document_variable_name = "context"
                llm = OpenAI()
                # The prompt here should take as an input variable the
                # `document_variable_name`
                prompt = PromptTemplate.from_template(
                    "Summarize this content: {context}"
                )
                llm_chain = LLMChain(llm=llm, prompt=prompt)
                combine_documents_chain = StuffDocumentsChain(
                    llm_chain=llm_chain,
                    document_prompt=document_prompt,
                    document_variable_name=document_variable_name
                )
                chain = ReduceDocumentsChain(
                    combine_documents_chain=combine_documents_chain,
                )
                # If we wanted to, we could also pass in collapse_documents_chain
                # which is specifically aimed at collapsing documents BEFORE
                # the final call.
                prompt = PromptTemplate.from_template(
                    "Collapse this content: {context}"
                )
                llm_chain = LLMChain(llm=llm, prompt=prompt)
                collapse_documents_chain = StuffDocumentsChain(
                    llm_chain=llm_chain,
                    document_prompt=document_prompt,
                    document_variable_name=document_variable_name
                )
                chain = ReduceDocumentsChain(
                    combine_documents_chain=combine_documents_chain,
                    collapse_documents_chain=collapse_documents_chain,
                )
        """

        combine_documents_chain: BaseCombineDocumentsChain
        """Final chain to call to combine documents.
        This is typically a StuffDocumentsChain."""
        collapse_documents_chain: Optional[BaseCombineDocumentsChain] = None
        """Chain to use to collapse documents if needed until they can all fit.
        If None, will use the combine_documents_chain.
        This is typically a StuffDocumentsChain."""
        token_max: int = 3000
        """The maximum number of tokens to group documents into. For example, if
        set to 3000 then documents will be grouped into chunks of no greater than
        3000 tokens before trying to combine them into a smaller chunk."""
        collapse_max_retries: Optional[int] = None
        """The maximum number of retries to collapse documents to fit token_max.
        If None, it will keep trying to collapse documents to fit token_max.
        Otherwise, after it reaches the max number, it will throw an error"""

        class Config:
            """Configuration for this pydantic object."""

            extra = Extra.forbid
            arbitrary_types_allowed = True

        @property
        def _collapse_chain(self) -> BaseCombineDocumentsChain:
            if self.collapse_documents_chain is not None:
                return self.collapse_documents_chain
            else:
                return self.combine_documents_chain

        def combine_docs(
            self,
            docs: List[Document],
            token_max: Optional[int] = None,
            callbacks: Callbacks = None,
            **kwargs: Any,
        ) -> Tuple[str, dict]:
            """Combine multiple documents recursively.

            Args:
                docs: List of documents to combine, assumed that each one is less than
                    `token_max`.
                token_max: Recursively creates groups of documents less than this number
                    of tokens.
                callbacks: Callbacks to be passed through
                **kwargs: additional parameters to be passed to LLM calls (like other
                    input variables besides the documents)

            Returns:
                The first element returned is the single string output. The second
                element returned is a dictionary of other keys to return.
            """
            result_docs, extra_return_dict = self._collapse(
                docs, token_max=token_max, callbacks=callbacks, **kwargs
            )
            return self.combine_documents_chain.combine_docs(
                docs=result_docs, callbacks=callbacks, **kwargs
            )

        async def acombine_docs(
            self,
            docs: List[Document],
            token_max: Optional[int] = None,
            callbacks: Callbacks = None,
            **kwargs: Any,
        ) -> Tuple[str, dict]:
            """Async combine multiple documents recursively.

            Args:
                docs: List of documents to combine, assumed that each one is less than
                    `token_max`.
                token_max: Recursively creates groups of documents less than this number
                    of tokens.
                callbacks: Callbacks to be passed through
                **kwargs: additional parameters to be passed to LLM calls (like other
                    input variables besides the documents)

            Returns:
                The first element returned is the single string output. The second
                element returned is a dictionary of other keys to return.
            """
            result_docs, extra_return_dict = await self._acollapse(
                docs, token_max=token_max, callbacks=callbacks, **kwargs
            )
            return await self.combine_documents_chain.acombine_docs(
                docs=result_docs, callbacks=callbacks, **kwargs
            )

        def _collapse(
            self,
            docs: List[Document],
            token_max: Optional[int] = None,
            callbacks: Callbacks = None,
            **kwargs: Any,
        ) -> Tuple[List[Document], dict]:
            result_docs = docs
            length_func = self.combine_documents_chain.prompt_length
            num_tokens = length_func(result_docs, **kwargs)

            def _collapse_docs_func(docs: List[Document], **kwargs: Any) -> str:
                return self._collapse_chain.run(
                    input_documents=docs, callbacks=callbacks, **kwargs
                )

            _token_max = token_max or self.token_max
            retries: int = 0
            while num_tokens is not None and num_tokens > _token_max:
                new_result_doc_list = split_list_of_docs(
                    result_docs, length_func, _token_max, **kwargs
                )
                result_docs = []
                for docs in new_result_doc_list:
                    new_doc = collapse_docs(docs, _collapse_docs_func, **kwargs)
                    result_docs.append(new_doc)
                num_tokens = length_func(result_docs, **kwargs)
                retries += 1
                if self.collapse_max_retries and retries == self.collapse_max_retries:
                    raise ValueError(
                        f"Exceed {self.collapse_max_retries} tries to \
                            collapse document to {_token_max} tokens."
                    )
            return result_docs, {}

        async def _acollapse(
            self,
            docs: List[Document],
            token_max: Optional[int] = None,
            callbacks: Callbacks = None,
            **kwargs: Any,
        ) -> Tuple[List[Document], dict]:
            result_docs = docs
            length_func = self.combine_documents_chain.prompt_length
            num_tokens = length_func(result_docs, **kwargs)

            async def _collapse_docs_func(docs: List[Document], **kwargs: Any) -> str:
                return await self._collapse_chain.arun(
                    input_documents=docs, callbacks=callbacks, **kwargs
                )

            _token_max = token_max or self.token_max
            retries: int = 0
            while num_tokens is not None and num_tokens > _token_max:
                new_result_doc_list = split_list_of_docs(
                    result_docs, length_func, _token_max, **kwargs
                )
                result_docs = []
                for docs in new_result_doc_list:
                    new_doc = await acollapse_docs(docs, _collapse_docs_func, **kwargs)
                    result_docs.append(new_doc)
                num_tokens = length_func(result_docs, **kwargs)
                retries += 1
                if self.collapse_max_retries and retries == self.collapse_max_retries:
                    raise ValueError(
                        f"Exceed {self.collapse_max_retries} tries to \
                            collapse document to {_token_max} tokens."
                    )
            return result_docs, {}

        @property
        def _chain_type(self) -> str:
            return "reduce_documents_chain"