Set empty_value to 'missing' for delimited fields

This was planned as a change to defaults in v9.0

Adjustments to the testsuite verify the effects of the change: a
DelimitedList will now use the `load_default` of a nested field when an
empty value (double-delimiter) appears.

Author: sirosen

CHANGELOG.rst

@@ -17,6 +17,11 @@ Other changes:
   Users should use ``await parser.async_parse()`` to access the async features
   of ``Parser``.
 
+* *Backwards-incompatible*: `DelimitedList` and `DelimitedTuple` fields have
+  changed their default `empty_value` from the empty string (`""`) to `missing`.
+  This allows nested fields with a `load_default` to be used to better customize
+  behavior.
+
 * Drop support for Python 3.9, which is EOL (:pr:`1019`).
 * Drop support for Bottle < 0.13 (:pr:`1019`).
 * Drop support for Flask < 3.1.0 (:pr:`1023`).

src/webargs/fields.py

@@ -42,8 +42,7 @@ class DelimitedFieldMixin:
     delimiter: str = ","
     # delimited fields set is_multiple=False for webargs.core.is_multiple
     is_multiple: bool = False
-    # NOTE: in 8.x this defaults to "" but in 9.x it will be 'missing'
-    empty_value: typing.Any = ""
+    empty_value: typing.Any = ma.missing
 
     def _serialize(self, value, attr, obj, **kwargs):
         # serializing will start with parent-class serialization, so that we correctly

tests/test_core.py

@@ -11,7 +11,6 @@
     INCLUDE,
     RAISE,
     Schema,
-    missing,
     pre_load,
     validates_schema,
 )
@@ -1119,21 +1118,30 @@ class ZeroTuple(fields.DelimitedTuple):
     assert parsed["ids"] == (1, 0, 3)
 
 
-def test_delimited_list_using_missing_for_empty(web_request, parser):
-    # this is "future" because we plan to make this the default for webargs v9.0
-    class FutureList(fields.DelimitedList):
-        empty_value = missing
-
+def test_delimited_list_missing_element_defaults_to_load_default(web_request, parser):
     web_request.json = {"ids": "foo,,bar"}
     schema_cls = Schema.from_dict(
-        {"ids": FutureList(fields.String(load_default="nil"))}
+        {"ids": fields.DelimitedList(fields.String(load_default="nil"))}
     )
     schema = schema_cls()
 
     parsed = parser.parse(schema, web_request)
     assert parsed["ids"] == ["foo", "nil", "bar"]
 
 
+def test_delimited_list_using_empty_string_for_empty(web_request, parser):
+    # this is "past" because it was the default for webargs v8.x
+    class PastList(fields.DelimitedList):
+        empty_value = ""
+
+    web_request.json = {"ids": "foo,,bar"}
+    schema_cls = Schema.from_dict({"ids": PastList(fields.String(load_default="nil"))})
+    schema = schema_cls()
+
+    parsed = parser.parse(schema, web_request)
+    assert parsed["ids"] == ["foo", "", "bar"]
+
+
 def test_missing_list_argument_not_in_parsed_result(web_request, parser):
     # arg missing in request
     web_request.json = {}