From c6484dd8206fdbc7d0536dfcbe59e7adb2947fc7 Mon Sep 17 00:00:00 2001
From: Kadir Can Ozden <101993364+bysiber@users.noreply.github.com>
Date: Sun, 22 Feb 2026 02:47:43 +0300
Subject: [PATCH 1/2] Fix ErrorTree polluting contents on index access

ErrorTree used a defaultdict for _contents, which meant accessing
a non-existent index via __getitem__ would silently create an empty
entry. This caused __iter__ and __contains__ to return incorrect
results after accessing an index with no errors.

Replaced defaultdict with a regular dict and changed __init__ to
explicitly create subtrees. __getitem__ now returns a new empty
ErrorTree for valid indices without storing it in _contents.
---
 commit_msg.txt                      | 10 ++++++++++
 jsonschema/exceptions.py            | 12 ++++++++----
 jsonschema/tests/test_exceptions.py | 17 +++++++++++++++++
 3 files changed, 35 insertions(+), 4 deletions(-)
 create mode 100644 commit_msg.txt

diff --git a/commit_msg.txt b/commit_msg.txt
new file mode 100644
index 00000000..493fcbe7
--- /dev/null
+++ b/commit_msg.txt
@@ -0,0 +1,10 @@
+Fix ErrorTree polluting contents on index access
+
+ErrorTree used a defaultdict for _contents, which meant accessing
+a non-existent index via __getitem__ would silently create an empty
+entry. This caused __iter__ and __contains__ to return incorrect
+results after accessing an index with no errors.
+
+Replaced defaultdict with a regular dict and changed __init__ to
+explicitly create subtrees. __getitem__ now returns a new empty
+ErrorTree for valid indices without storing it in _contents.
diff --git a/jsonschema/exceptions.py b/jsonschema/exceptions.py
index 2e5d4ca0..4bc9349a 100644
--- a/jsonschema/exceptions.py
+++ b/jsonschema/exceptions.py
@@ -3,7 +3,7 @@
 """
 from __future__ import annotations
 
-from collections import defaultdict, deque
+from collections import deque
 from pprint import pformat
 from textwrap import dedent, indent
 from typing import TYPE_CHECKING, Any, ClassVar
@@ -321,12 +321,14 @@ class ErrorTree:
 
     def __init__(self, errors: Iterable[ValidationError] = ()):
         self.errors: MutableMapping[str, ValidationError] = {}
-        self._contents: Mapping[str, ErrorTree] = defaultdict(self.__class__)
+        self._contents: dict[str | int, ErrorTree] = {}
 
         for error in errors:
             container = self
             for element in error.path:
-                container = container[element]
+                if element not in container._contents:
+                    container._contents[element] = self.__class__()
+                container = container._contents[element]
             container.errors[error.validator] = error
 
             container._instance = error.instance
@@ -348,7 +350,9 @@ def __getitem__(self, index):
         """
         if self._instance is not _unset and index not in self:
             self._instance[index]
-        return self._contents[index]
+        if index in self._contents:
+            return self._contents[index]
+        return self.__class__()
 
     def __setitem__(self, index: str | int, value: ErrorTree):
         """
diff --git a/jsonschema/tests/test_exceptions.py b/jsonschema/tests/test_exceptions.py
index 358b9242..f4a99cbf 100644
--- a/jsonschema/tests/test_exceptions.py
+++ b/jsonschema/tests/test_exceptions.py
@@ -506,6 +506,23 @@ def test_repr_empty(self):
         tree = exceptions.ErrorTree([])
         self.assertEqual(repr(tree), "<ErrorTree (0 total errors)>")
 
+    def test_accessing_index_without_error_does_not_pollute_contents(self):
+        error = exceptions.ValidationError(
+            "not a number",
+            validator="type",
+            path=[0],
+            instance=["spam", 2],
+        )
+        tree = exceptions.ErrorTree([error])
+        self.assertEqual(list(tree), [0])
+        self.assertNotIn(1, tree)
+
+        # accessing an index with no error should not add it to the tree
+        tree[1]
+
+        self.assertEqual(list(tree), [0])
+        self.assertNotIn(1, tree)
+
 
 class TestErrorInitReprStr(TestCase):
     def make_error(self, **kwargs):

From 23c8c8b979690a57fcab84ed775e1609409ef7fd Mon Sep 17 00:00:00 2001
From: Kadir Can Ozden <101993364+bysiber@users.noreply.github.com>
Date: Sun, 22 Feb 2026 17:44:19 +0300
Subject: [PATCH 2/2] Delete commit_msg.txt

---
 commit_msg.txt | 10 ----------
 1 file changed, 10 deletions(-)
 delete mode 100644 commit_msg.txt

diff --git a/commit_msg.txt b/commit_msg.txt
deleted file mode 100644
index 493fcbe7..00000000
--- a/commit_msg.txt
+++ /dev/null
@@ -1,10 +0,0 @@
-Fix ErrorTree polluting contents on index access
-
-ErrorTree used a defaultdict for _contents, which meant accessing
-a non-existent index via __getitem__ would silently create an empty
-entry. This caused __iter__ and __contains__ to return incorrect
-results after accessing an index with no errors.
-
-Replaced defaultdict with a regular dict and changed __init__ to
-explicitly create subtrees. __getitem__ now returns a new empty
-ErrorTree for valid indices without storing it in _contents.