Fix lost 0. index prefix for ListSerializer errors on DRF 3.18

docs/changelog.md

@@ -5,6 +5,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [UNRELEASED]
+### Fixed
+- Fix the `attr` of errors raised on the first item (index `0`) of a `many=True` list serializer losing its `0.`
+  index prefix with DRF 3.18, since `ListSerializer.to_internal_value` now collects child errors in a dict keyed by
+  the integer index of the item, and integer `0` was being treated as falsy.
 
 ## [0.16.0] - 2026-04-29
 ### Added

drf_standardized_errors/formatter.py

@@ -135,7 +135,7 @@ def flatten_errors(
 
         elif isinstance(detail, dict):
             for key, value in detail.items():
-                if attr:
+                if attr is not None:
                     key = f"{attr}{package_settings.NESTED_FIELD_SEPARATOR}{key}"
                 fifo.append((value, key, None))
 

tests/test_flatten_errors.py

@@ -126,3 +126,22 @@ def test_exception_with_detail_empty():
     assert len(errors) == 1
     assert errors[0].attr == "some_field"
     assert errors[0].detail == ""
+
+
+@pytest.fixture
+def list_serializer_errors_indexed_dict():
+    # since DRF 3.18, ListSerializer.to_internal_value collects child errors in a
+    # dict keyed by the integer index of the item instead of a positional list.
+    return {
+        0: {"email": [ErrorDetail("Enter a valid email address.", code="invalid")]},
+        1: {"email": [ErrorDetail("Enter a valid email address.", code="invalid")]},
+    }
+
+
+def test_list_serializer_errors_indexed_dict(list_serializer_errors_indexed_dict):
+    errors = flatten_errors(list_serializer_errors_indexed_dict)
+    assert len(errors) == 2
+    assert errors[0].code == "invalid"
+    assert errors[0].attr == "0.email"
+    assert errors[1].code == "invalid"
+    assert errors[1].attr == "1.email"