Export item fields in declaration order (#7824)

docs/topics/exporters.rst

@@ -211,6 +211,16 @@ BaseItemExporter
 
       -   ``None`` (all fields [2]_, default)
 
+          Fields are exported in declaration order, i.e. the order in which
+          they are defined in the :ref:`item class <item-types>`. For
+          :class:`dict` items, which have no declared fields, the key order of
+          each item is used instead.
+
+          .. versionchanged:: VERSION
+             Fields of non-\ :class:`dict` items used to be exported in the
+             order in which they had been populated, except in
+             :class:`CsvItemExporter`, which has always used declaration order.
+
       -   A list of fields:
 
           .. code-block:: python

scrapy/exporters.py

@@ -74,6 +74,17 @@ class BaseItemExporter(ABC):
     def finish_exporting(self) -> None:  # noqa: B027
         pass
 
+    @staticmethod
+    def _get_populated_field_names(adapter: ItemAdapter) -> Iterable[str]:
+        """Return the populated field names of *adapter*, in declaration order.
+
+        Populated fields that are not declared, which some item types allow,
+        come last, in item order.
+        """
+        populated = set(adapter.keys())
+        declared = (name for name in adapter.field_names() if name in populated)
+        return dict.fromkeys([*declared, *adapter.keys()])
+
     def _get_serialized_fields(
         self, item: Any, default_value: Any = None, include_empty: bool | None = None
     ) -> Iterable[tuple[str, Any]]:
@@ -86,7 +97,11 @@ class BaseItemExporter(ABC):
             include_empty = self.export_empty_fields
 
         if self.fields_to_export is None:
-            field_iter = item.field_names() if include_empty else item.keys()
+            field_iter = (
+                item.field_names()
+                if include_empty
+                else self._get_populated_field_names(item)
+            )
         elif isinstance(self.fields_to_export, Mapping):
             if include_empty:
                 field_iter = self.fields_to_export.items()

tests/test_exporters.py

@@ -118,6 +118,18 @@ class TestBaseItemExporter(ABC):
         ie = self._get_exporter(fields_to_export={"name": "名稱"})
         assert list(ie._get_serialized_fields(self.i)) == [("名稱", "John\xa3")]
 
+    def test_field_order(self):
+        item = self.item_class(age="22", name="John\xa3")
+        ie = self._get_exporter()
+        assert [name for name, _ in ie._get_serialized_fields(item)] == ["name", "age"]
+
+    def test_field_order_dict_item(self):
+        ie = self._get_exporter()
+        assert [name for name, _ in ie._get_serialized_fields({"age": "22"})] == ["age"]
+        assert [
+            name for name, _ in ie._get_serialized_fields({"age": "22", "name": "John"})
+        ] == ["age", "name"]
+
     def test_field_custom_serializer(self):
         i = self.custom_field_item_class(name="John\xa3", age="22")
         a = ItemAdapter(i)

tests/test_feedexport.py

@@ -675,14 +675,14 @@ class TestFeedExport(TestFeedExportBase):
 
         formats = {
             "csv": b"foo,egg,baz\r\nbar1,spam1,\r\n",
-            "json": b'[\n{"hello": "world2", "foo": "bar2"}\n]',
+            "json": b'[\n{"foo": "bar2", "hello": "world2"}\n]',
             "jsonlines": (
-                b'{"foo": "bar1", "egg": "spam1"}\n{"hello": "world2", "foo": "bar2"}\n'
+                b'{"foo": "bar1", "egg": "spam1"}\n{"foo": "bar2", "hello": "world2"}\n'
             ),
             "xml": (
                 b'<?xml version="1.0" encoding="utf-8"?>\n<items>\n<item>'
-                b"<foo>bar1</foo><egg>spam1</egg></item>\n<item><hello>"
-                b"world2</hello><foo>bar2</foo></item>\n<item><hello>world3"
+                b"<foo>bar1</foo><egg>spam1</egg></item>\n<item><foo>"
+                b"bar2</foo><hello>world2</hello></item>\n<item><hello>world3"
                 b"</hello><egg>spam3</egg></item>\n</items>"
             ),
         }
@@ -740,8 +740,8 @@ class TestFeedExport(TestFeedExportBase):
             "json": b'[\n{"foo": "bar1", "egg": "spam1"}\n]',
             "xml": (
                 b'<?xml version="1.0" encoding="utf-8"?>\n<items>\n<item>'
-                b"<foo>bar1</foo><egg>spam1</egg></item>\n<item><hello>"
-                b"world2</hello><foo>bar2</foo></item>\n</items>"
+                b"<foo>bar1</foo><egg>spam1</egg></item>\n<item><foo>"
+                b"bar2</foo><hello>world2</hello></item>\n</items>"
             ),
             "jsonlines": b'{"foo": "bar1", "egg": "spam1"}\n',
         }