Add BlobParser abstraction (#3979)

This PR adds the BlobParser abstraction.

It follows the proposal described here:
https://github.com/hwchase17/langchain/pull/2833#issuecomment-1509097756

langchain/document_loaders/base.py

@@ -1,9 +1,10 @@
 """Abstract interface for document loader implementations."""
-
+import abc
 from abc import ABC, abstractmethod
-from typing import Iterable, List, Optional
+from typing import Iterable, Iterator, List, Optional
 
-from langchain.docstore.document import Document
+from langchain.document_loaders.blob_loaders import Blob
+from langchain.schema import Document
 from langchain.text_splitter import RecursiveCharacterTextSplitter, TextSplitter
 
 
@@ -44,3 +45,44 @@ class BaseLoader(ABC):
         raise NotImplementedError(
             f"{self.__class__.__name__} does not implement lazy_load()"
         )
+
+
+class BaseBlobParser(abc.ABC):
+    """Abstract interface for blob parsers.
+
+    A blob parser is provides a way to parse raw data stored in a blob into one
+    or more documents.
+
+    The parser can be composed with blob loaders, making it easy to re-use
+    a parser independent of how the blob was originally loaded.
+    """
+
+    @abc.abstractmethod
+    def lazy_parse(self, blob: Blob) -> Iterator[Document]:
+        """Lazy parsing interface.
+
+        Subclasses are required to implement this method.
+
+        Args:
+            blob: Blob instance
+
+        Returns:
+            Generator of documents
+        """
+
+    def parse(self, blob: Blob) -> List[Document]:
+        """Eagerly parse the blob into a document or documents.
+
+        This is a convenience method for interactive development environment.
+
+        Production applications should favor the lazy_parse method instead.
+
+        Subclasses should generally not over-ride this parse method.
+
+        Args:
+            blob: Blob instance
+
+        Returns:
+            List of documents
+        """
+        return list(self.lazy_parse(blob))

tests/unit_tests/document_loader/test_base.py

@@ -0,0 +1,28 @@
+"""Test Base Schema of documents."""
+from typing import Iterator
+
+from langchain.document_loaders.base import BaseBlobParser
+from langchain.document_loaders.blob_loaders import Blob
+from langchain.schema import Document
+
+
+def test_base_blob_parser() -> None:
+    """Verify that the eager method is hooked up to the lazy method by default."""
+
+    class MyParser(BaseBlobParser):
+        """A simple parser that returns a single document."""
+
+        def lazy_parse(self, blob: Blob) -> Iterator[Document]:
+            """Lazy parsing interface."""
+            yield Document(
+                page_content="foo",
+            )
+
+    parser = MyParser()
+
+    assert isinstance(parser.lazy_parse(Blob(data="who?")), Iterator)
+
+    # We're verifying that the eager method is hooked up to the lazy method by default.
+    docs = parser.parse(Blob(data="who?"))
+    assert len(docs) == 1
+    assert docs[0].page_content == "foo"