From 7d18a2eb1dd50318d43df5693e784c49fcd86490 Mon Sep 17 00:00:00 2001
From: Stephen Finucane <stephen@that.guru>
Date: Wed, 5 Mar 2025 14:52:24 +0000
Subject: [PATCH 1/2] Raise exception if validator mutates instance

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #1338
---
 jsonschema/exceptions.py | 14 ++++++++++++++
 jsonschema/validators.py | 11 +++++++++++
 2 files changed, 25 insertions(+)

diff --git a/jsonschema/exceptions.py b/jsonschema/exceptions.py
index 3dcd29667..7278180fe 100644
--- a/jsonschema/exceptions.py
+++ b/jsonschema/exceptions.py
@@ -292,6 +292,20 @@ def __str__(self):
         )
 
 
+class MutatingValidator(Exception):
+    """
+    A validator mutated the instance.
+    """
+    def __init__(self, validator: Any) -> None:
+        self.validator = validator
+
+    def __str__(self) -> str:
+        return (
+            f"Validator {self.validator!r} mutated the instance. "
+            f"This can cause failures for later validators."
+        )
+
+
 class FormatError(Exception):
     """
     Validating a format failed.
diff --git a/jsonschema/validators.py b/jsonschema/validators.py
index b8ca3bd45..991bf74c7 100644
--- a/jsonschema/validators.py
+++ b/jsonschema/validators.py
@@ -5,6 +5,7 @@
 
 from collections import deque
 from collections.abc import Iterable, Mapping, Sequence
+import copy
 from functools import lru_cache
 from operator import methodcaller
 from typing import TYPE_CHECKING
@@ -380,7 +381,12 @@ def iter_errors(self, instance, _schema=None):
                 return
 
             for validator, k, v in validators:
+                instance_clone = copy.deepcopy(instance)
                 errors = validator(self, v, instance, _schema) or ()
+                if instance != instance_clone:
+                    yield exceptions.MutatingValidator(validator)
+                del instance_clone
+
                 for error in errors:
                     # set details if not already set by the called fn
                     error._set(
@@ -428,7 +434,12 @@ def descend(
                 if validator is None:
                     continue
 
+                instance_clone = copy.deepcopy(instance)
                 errors = validator(evolved, v, instance, schema) or ()
+                if instance != instance_clone:
+                    yield exceptions.MutatingValidator(validator)
+                del instance_clone
+
                 for error in errors:
                     # set details if not already set by the called fn
                     error._set(

From 46a083c702ffa5ddadc0a1c94ec5adad5f02d358 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Wed, 5 Mar 2025 15:29:20 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 jsonschema/exceptions.py | 1 +
 jsonschema/validators.py | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/jsonschema/exceptions.py b/jsonschema/exceptions.py
index 7278180fe..089546a7d 100644
--- a/jsonschema/exceptions.py
+++ b/jsonschema/exceptions.py
@@ -296,6 +296,7 @@ class MutatingValidator(Exception):
     """
     A validator mutated the instance.
     """
+
     def __init__(self, validator: Any) -> None:
         self.validator = validator
 
diff --git a/jsonschema/validators.py b/jsonschema/validators.py
index 991bf74c7..a8e270569 100644
--- a/jsonschema/validators.py
+++ b/jsonschema/validators.py
@@ -5,7 +5,6 @@
 
 from collections import deque
 from collections.abc import Iterable, Mapping, Sequence
-import copy
 from functools import lru_cache
 from operator import methodcaller
 from typing import TYPE_CHECKING
@@ -13,6 +12,7 @@
 from urllib.request import urlopen
 from warnings import warn
 import contextlib
+import copy
 import json
 import reprlib
 import warnings