From 0f7c796275a0c8c3a2020de8f1a26d138628b02c Mon Sep 17 00:00:00 2001
From: Sergei Shishov <sergei.shishov@wafeq.com>
Date: Wed, 2 Sep 2026 16:27:01 +0400
Subject: [PATCH] Fix lost 0. index prefix for ListSerializer errors on DRF
 3.18

Since DRF 3.18, ListSerializer.to_internal_value collects child
validation errors in a dict keyed by the integer index of the item
(e.g. {0: {"email": [...]}}) instead of a positional list. In
flatten_errors, the dict branch only prefixed a child key with the
parent attr when `if attr:` was truthy. Since the integer 0 is
falsy in Python, errors on the first item of a many=True payload
lost their "0." prefix, while items at index 1 and above were
prefixed correctly (any other int is truthy).

Fix by checking `attr is not None` instead of relying on the
truthiness of attr, so an attr of 0 is still treated as a valid
prefix segment.

Fixes ghazi-git/drf-standardized-errors#121
---
 docs/changelog.md                    |  4 ++++
 drf_standardized_errors/formatter.py |  2 +-
 tests/test_flatten_errors.py         | 19 +++++++++++++++++++
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/docs/changelog.md b/docs/changelog.md
index c37838e..24b8c01 100644
--- a/docs/changelog.md
+++ b/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
diff --git a/drf_standardized_errors/formatter.py b/drf_standardized_errors/formatter.py
index f51c374..32e9ecd 100644
--- a/drf_standardized_errors/formatter.py
+++ b/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))
 
diff --git a/tests/test_flatten_errors.py b/tests/test_flatten_errors.py
index 92d58c9..19fd7f1 100644
--- a/tests/test_flatten_errors.py
+++ b/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"