Raise exception if validator mutates instance

jsonschema/exceptions.py

@@ -292,6 +292,21 @@ 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.

jsonschema/validators.py

@@ -12,6 +12,7 @@
 from urllib.request import urlopen
 from warnings import warn
 import contextlib
+import copy
 import json
 import reprlib
 import warnings
@@ -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(