From f42ad69366e8854186904df9bdd51c854e30c5ff Mon Sep 17 00:00:00 2001
From: Umut Alisoglu <umutalisoglu@gmail.com>
Date: Sun, 25 Oct 2026 22:13:37 +0100
Subject: [PATCH] Preserve empty-string parent keys when flattening

---
 benedict/core/flatten.py     | 10 ++++++----
 tests/core/test_flatten.py   |  4 ++--
 tests/dicts/test_benedict.py | 22 +++++++++++++++++++++-
 3 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/benedict/core/flatten.py b/benedict/core/flatten.py
index f27e16f8..ac6e08dd 100644
--- a/benedict/core/flatten.py
+++ b/benedict/core/flatten.py
@@ -1,11 +1,13 @@
+from __future__ import annotations
+
 from collections.abc import Mapping
 from typing import Any
 
 from benedict.core.clone import clone
 
 
-def _flatten_key(base_key: str, key: str, separator: str) -> str:
-    if base_key and separator:
+def _flatten_key(base_key: str | None, key: str, separator: str) -> str:
+    if base_key is not None and separator:
         return f"{base_key}{separator}{key}"
     return key
 
@@ -49,7 +51,7 @@ def _flatten_list(
 def _flatten_item(
     d: Any,
     base_dict: Any,
-    base_key: str,
+    base_key: str | None,
     separator: str,
     indexes: bool = False,
 ) -> Any:
@@ -87,7 +89,7 @@ def flatten(d: Any, separator: str = "_", indexes: bool = False) -> Any:
     # collect items in a plain dict, flattened keys (eg. "a[0]")
     # must not be parsed as keypaths by benedict instances
     items = _flatten_item(
-        d, base_dict={}, base_key="", separator=separator, indexes=indexes
+        d, base_dict={}, base_key=None, separator=separator, indexes=indexes
     )
     new_dict = clone(d, empty=True)
     new_dict.update(items)
diff --git a/tests/core/test_flatten.py b/tests/core/test_flatten.py
index 52c05005..2436c0a7 100644
--- a/tests/core/test_flatten.py
+++ b/tests/core/test_flatten.py
@@ -33,7 +33,7 @@ def test_flatten(self) -> None:
         self.assertEqual(o, r)
 
     def test_flatten_with_falsy_parent_keys(self) -> None:
-        for key in (0, False, None):
+        for key in (0, False, None, ""):
             for separator in ("_", "/"):
                 for indexes in (False, True):
                     with self.subTest(key=key, separator=separator, indexes=indexes):
@@ -45,7 +45,7 @@ def test_flatten_with_falsy_parent_keys(self) -> None:
                         self.assertEqual(data, {key: {"child": 1}, "child": 2})
 
     def test_flatten_with_falsy_parent_keys_and_indexed_lists(self) -> None:
-        for key in (0, False, None):
+        for key in (0, False, None, ""):
             with self.subTest(key=key):
                 self.assertEqual(
                     _flatten({key: {"items": [{"value": 1}]}}, indexes=True),
diff --git a/tests/dicts/test_benedict.py b/tests/dicts/test_benedict.py
index c2db8375..b38a42a0 100644
--- a/tests/dicts/test_benedict.py
+++ b/tests/dicts/test_benedict.py
@@ -524,7 +524,7 @@ def test_flatten(self) -> None:
         self.assertTrue(isinstance(f, benedict))
 
     def test_flatten_with_falsy_parent_keys(self) -> None:
-        for key in (0, False, None):
+        for key in (0, False, None, ""):
             with self.subTest(key=key):
                 source = benedict({key: {"child": 1}, "child": 2})
                 result = source.flatten()
@@ -532,6 +532,26 @@ def test_flatten_with_falsy_parent_keys(self) -> None:
                 self.assertIsInstance(result, benedict)
                 self.assertEqual(source, {key: {"child": 1}, "child": 2})
 
+    def test_flatten_empty_parent_keys_round_trip(self) -> None:
+        for separator in ("_", "/"):
+            with self.subTest(separator=separator):
+                data = {"": {"": 1, "items": [{"value": 2}]}, "value": 3}
+                source = benedict(data)
+                result = source.flatten(separator=separator, indexes=True)
+                self.assertEqual(
+                    result,
+                    {
+                        separator: 1,
+                        f"{separator}items[0]{separator}value": 2,
+                        "value": 3,
+                    },
+                )
+                self.assertIsInstance(result, benedict)
+                self.assertEqual(result.unflatten(separator=separator), data)
+                self.assertEqual(
+                    data, {"": {"": 1, "items": [{"value": 2}]}, "value": 3}
+                )
+
     def test_flatten_with_custom_keypath_separator(self) -> None:
         d = {
             "a": 1,