Add an item_processor feed option

docs/topics/feed-exports.rst

@@ -338,6 +338,35 @@ ItemFilter
    :members:
 
 
+.. _item-processor:
+
+Item processing
+===============
+
+.. versionadded:: VERSION
+
+The ``item_processor`` :ref:`feed option <feed-options>` takes a callable, or
+its import path, that receives an accepted item and returns an iterable of the
+items to export in its place:
+
+.. code-block:: python
+
+    def split_variants(item):
+        for variant in item["variants"]:
+            yield {**item, "variants": None, **variant}
+
+Returning an empty iterable drops the item from that feed, and returning more
+than one item writes one entry per returned item.
+
+Item processors run after :ref:`item filtering <item-filter>`, and only affect
+the feed that declares them. Items are exported as returned, so the
+:signal:`item_scraped` signal, :ref:`item pipelines <topics-item-pipeline>` and
+the ``item_scraped_count`` stat still see the item as scraped; use item
+processors for output formatting, and item pipelines for anything that should
+apply to the item itself. The number of exported entries per feed is reported
+as the ``feedexport/item_count/<storage class>`` stat.
+
+
 .. _post-processing:
 
 Post-Processing
@@ -490,6 +519,13 @@ as a fallback value if that key is not provided for a specific feed definition:
 
     :class:`~scrapy.extensions.feedexport.ItemFilter` is used be default.
 
+-   ``item_processor``: an :ref:`item processor <item-processor>` to reshape
+    items before they are exported.
+
+    .. versionadded:: VERSION
+
+    If undefined, items are exported as scraped.
+
 -   ``indent``: falls back to :setting:`FEED_EXPORT_INDENT`.
 
 -   ``item_export_kwargs``: :class:`dict` with keyword arguments for the corresponding :ref:`item exporter class <topics-exporters>`.

scrapy/extensions/feedexport.py

@@ -13,7 +13,7 @@ import re
 import sys
 import warnings
 from abc import ABC, abstractmethod
-from collections.abc import Callable
+from collections.abc import Callable, Iterable
 from datetime import datetime, timezone
 from pathlib import Path, PureWindowsPath
 from tempfile import NamedTemporaryFile
@@ -79,6 +79,8 @@ UriParamsCallableT: TypeAlias = Callable[
     [dict[str, Any], Spider], dict[str, Any] | None
 ]
 
+_ItemProcessor: TypeAlias = Callable[[Any], Iterable[Any]]
+
 
 class ItemFilter:
     """
@@ -486,6 +488,7 @@ class FeedExporter:
         self.feeds = {}
         self.slots: list[FeedSlot] = []
         self.filters: dict[str, ItemFilter] = {}
+        self.processors: dict[str, _ItemProcessor | None] = {}
         self._pending_close_tasks: list[asyncio.Task[None] | Deferred[None]] = []
 
         if not self.settings["FEEDS"] and not self.settings["FEED_URI"]:
@@ -507,6 +510,7 @@ class FeedExporter:
                 feed_options, self.settings
             )
             self.filters[uri] = self._load_filter(feed_options)
+            self.processors[uri] = self._load_processor(feed_options)
         # End: Backward compatibility for FEED_URI and FEED_FORMAT settings
 
         # 'FEEDS' setting takes precedence over 'FEED_URI'
@@ -521,6 +525,7 @@ class FeedExporter:
                 feed_options, self.settings
             )
             self.filters[uri] = self._load_filter(feed_options)
+            self.processors[uri] = self._load_processor(feed_options)
 
         self.storages: dict[str, type[FeedStorageProtocol]] = self._load_components(
             "FEED_STORAGES"
@@ -612,6 +617,9 @@ class FeedExporter:
         logmsg = f"{slot.format} feed ({slot.itemcount} items) in: {slot.uri}"
         slot_type = type(slot.storage).__name__
         assert self.crawler.stats
+        self.crawler.stats.inc_value(
+            f"feedexport/item_count/{slot_type}", slot.itemcount
+        )
         try:
             await ensure_awaitable(slot.storage.store(self._get_file(slot)))
         except Exception:
@@ -672,30 +680,30 @@ class FeedExporter:
                 )  # if slot doesn't accept item, continue with next slot
                 continue
 
-            slot.start_exporting()
-            assert slot.exporter
-            slot.exporter.export_item(item)
-            slot.itemcount += 1
-            # create new slot for each slot with itemcount == FEED_EXPORT_BATCH_ITEM_COUNT and close the old one
-            if (
-                self.feeds[slot.uri_template]["batch_item_count"]
-                and slot.itemcount >= self.feeds[slot.uri_template]["batch_item_count"]
-            ):
-                uri_params = self._get_uri_params(
-                    spider, self.feeds[slot.uri_template]["uri_params"], slot
-                )
-                self._schedule_slot_close(slot, spider)
-                slots.append(
-                    self._start_new_batch(
+            processor = self.processors[slot.uri_template]
+            for exported_item in processor(item) if processor else (item,):
+                slot.start_exporting()
+                assert slot.exporter
+                slot.exporter.export_item(exported_item)
+                slot.itemcount += 1
+                # create new slot for each slot with itemcount == FEED_EXPORT_BATCH_ITEM_COUNT and close the old one
+                if (
+                    self.feeds[slot.uri_template]["batch_item_count"]
+                    and slot.itemcount
+                    >= self.feeds[slot.uri_template]["batch_item_count"]
+                ):
+                    uri_params = self._get_uri_params(
+                        spider, self.feeds[slot.uri_template]["uri_params"], slot
+                    )
+                    self._schedule_slot_close(slot, spider)
+                    slot = self._start_new_batch(  # noqa: PLW2901
                         batch_id=slot.batch_id + 1,
                         uri=apply_uri_params(slot.uri_template, uri_params),
                         feed_options=self.feeds[slot.uri_template],
                         spider=spider,
                         uri_template=slot.uri_template,
                     )
-                )
-            else:
-                slots.append(slot)
+            slots.append(slot)
         self.slots = slots
 
     def _load_components(self, setting_prefix: str) -> dict[str, Any]:
@@ -781,6 +789,10 @@ class FeedExporter:
         )
         return item_filter_class(feed_options)
 
+    def _load_processor(self, feed_options: dict[str, Any]) -> _ItemProcessor | None:
+        item_processor = feed_options.get("item_processor")
+        return load_object(item_processor) if item_processor else None
+
 
 def __getattr__(name: str) -> Any:  # pragma: no cover
     if name == "IFeedStorage":

tests/test_feedexport.py

@@ -140,6 +140,15 @@ class ExceptionJsonItemExporter(JsonItemExporter):
         raise RuntimeError("foo")
 
 
+def split_foo(item):
+    for value in item["foo"].split(","):
+        yield {"foo": value}
+
+
+def drop_item(item):
+    return []
+
+
 class TestFeedExport(TestFeedExportBase):
     async def run_and_export(
         self, spider_cls: type[Spider], settings: dict[str, Any]
@@ -769,6 +778,72 @@ class TestFeedExport(TestFeedExportBase):
         for fmt, expected in formats.items():
             assert data[fmt] == expected
 
+    @coroutine_test
+    async def test_export_based_on_item_processors(self):
+        items = [
+            MyItem({"foo": "bar1,bar2"}),
+            {"foo": "bar3"},
+        ]
+
+        formats = {
+            "jsonlines": b'{"foo": "bar1"}\n{"foo": "bar2"}\n{"foo": "bar3"}\n',
+            "json": b'[\n{"foo": "bar1"},\n{"foo": "bar2"},\n{"foo": "bar3"}\n]',
+            "xml": (
+                b'<?xml version="1.0" encoding="utf-8"?>\n<items>\n'
+                b"<item><foo>bar1</foo></item>\n<item><foo>bar2</foo></item>\n</items>"
+            ),
+            "csv": b"",
+        }
+
+        settings = {
+            "FEEDS": {
+                self._random_temp_filename(): {
+                    "format": "jsonlines",
+                    "item_processor": split_foo,
+                },
+                self._random_temp_filename(): {
+                    "format": "json",
+                    "item_processor": "tests.test_feedexport.split_foo",
+                },
+                self._random_temp_filename(): {
+                    "format": "xml",
+                    "item_classes": [MyItem],
+                    "item_processor": split_foo,
+                },
+                self._random_temp_filename(): {
+                    "format": "csv",
+                    "item_processor": drop_item,
+                },
+            },
+        }
+
+        data = await self.exported_data(items, settings)
+        for fmt, expected in formats.items():
+            assert data[fmt] == expected
+
+    @coroutine_test
+    async def test_item_processor_stats(self):
+        class TestSpider(scrapy.Spider):
+            name = "testspider"
+            start_urls = [self.mockserver.url("/")]
+
+            def parse(self, response):
+                yield {"foo": "bar1,bar2"}
+
+        settings = {
+            "FEEDS": {
+                path_to_url(self._random_temp_filename()): {
+                    "format": "jsonlines",
+                    "item_processor": split_foo,
+                },
+            },
+        }
+        crawler = get_crawler(TestSpider, settings)
+        await crawler.crawl_async()
+
+        assert crawler.stats.get_value("item_scraped_count") == 1
+        assert crawler.stats.get_value("feedexport/item_count/FileFeedStorage") == 2
+
     @coroutine_test
     async def test_export_dicts(self):
         # When dicts are used, only keys from the first row are used as

tests/test_feedexport_batch.py

@@ -383,6 +383,36 @@ class TestBatchDeliveries(TestFeedExportBase):
             for expected_batch, got_batch in zip(expected, data[fmt], strict=True):
                 assert got_batch == expected_batch
 
+    @coroutine_test
+    async def test_batch_item_count_with_item_processor(self):
+        def split_foo(item):
+            for value in item["foo"].split(","):
+                yield {"foo": value}
+
+        items = [{"foo": "FOO,FOO1"}, {"foo": "FOO2"}]
+        formats = {
+            "json": [
+                b'[{"foo": "FOO"}]',
+                b'[{"foo": "FOO1"}]',
+                b'[{"foo": "FOO2"}]',
+            ],
+        }
+        settings = {
+            "FEEDS": {
+                self._random_temp_filename() / "json" / self._file_mark: {
+                    "format": "json",
+                    "indent": None,
+                    "encoding": "utf-8",
+                    "batch_item_count": 1,
+                    "item_processor": split_foo,
+                },
+            },
+        }
+        data = await self.exported_data(items, settings)
+        for fmt, expected in formats.items():
+            for expected_batch, got_batch in zip(expected, data[fmt], strict=True):
+                assert got_batch == expected_batch
+
     @coroutine_test
     async def test_batch_path_differ(self):
         """