community[patch]: added content_columns option to CSVLoader (#23809)

**Description:** 
Adding a new option to the CSVLoader that allows us to implicitly
specify the columns that are used for generating the Document content.
Currently these are implicitly set as "all fields not part of the
metadata_columns".

In some cases however it is useful to have a field both as a metadata
and as part of the document content.

libs/community/langchain_community/document_loaders/csv_loader.py

@@ -104,6 +104,8 @@ class CSVLoader(BaseLoader):
         csv_args: Optional[Dict] = None,
         encoding: Optional[str] = None,
         autodetect_encoding: bool = False,
+        *,
+        content_columns: Sequence[str] = (),
     ):
         """
 
@@ -116,6 +118,8 @@ class CSVLoader(BaseLoader):
               Optional. Defaults to None.
             encoding: The encoding of the CSV file. Optional. Defaults to None.
             autodetect_encoding: Whether to try to autodetect the file encoding.
+            content_columns: A sequence of column names to use for the document content.
+                If not present, use all columns that are not part of the metadata.
         """
         self.file_path = file_path
         self.source_column = source_column
@@ -123,6 +127,7 @@ class CSVLoader(BaseLoader):
         self.encoding = encoding
         self.csv_args = csv_args or {}
         self.autodetect_encoding = autodetect_encoding
+        self.content_columns = content_columns
 
     def lazy_load(self) -> Iterator[Document]:
         try:
@@ -163,7 +168,11 @@ class CSVLoader(BaseLoader):
                 if isinstance(v, str) else ','.join(map(str.strip, v))
                 if isinstance(v, list) else v}"""
                 for k, v in row.items()
-                if k not in self.metadata_columns
+                if (
+                    k in self.content_columns
+                    if self.content_columns
+                    else k not in self.metadata_columns
+                )
             )
             metadata = {"source": source, "row": i}
             for col in self.metadata_columns:

libs/community/tests/unit_tests/document_loaders/test_csv_loader.py

@@ -108,6 +108,27 @@ class TestCSVLoader:
         # Assert
         assert result == expected_docs
 
+    def test_csv_loader_content_columns(self) -> None:
+        # Setup
+        file_path = self._get_csv_file_path("test_none_col.csv")
+        expected_docs = [
+            Document(
+                page_content="column1: value1\n" "column3: value3",
+                metadata={"source": file_path, "row": 0},
+            ),
+            Document(
+                page_content="column1: value6\n" "column3: value8",
+                metadata={"source": file_path, "row": 1},
+            ),
+        ]
+
+        # Exercise
+        loader = CSVLoader(file_path=file_path, content_columns=("column1", "column3"))
+        result = loader.load()
+
+        # Assert
+        assert result == expected_docs
+
     # utility functions
     def _get_csv_file_path(self, file_name: str) -> str:
         return str(Path(__file__).resolve().parent / "test_docs" / "csv" / file_name)