Copy non-callable Optional defaults so mutable defaults aren't shared (fixes #352)

schema/__init__.py

@@ -2,6 +2,7 @@
 obtained from config-files, forms, external services or command-line
 parsing, converted from JSON/YAML (or something else) to Python data-types."""
 
+import copy
 import inspect
 import re
 from typing import (
@@ -554,7 +555,9 @@ def validate(self, data: Any, **kwargs: Dict[str, Any]) -> Any:
                 new[default.key] = (
                     _invoke_with_optional_kwargs(default.default, **kwargs)
                     if callable(default.default)
-                    else default.default
+                    # Copy non-callable defaults so a mutable default (e.g. [] or
+                    # {}) is not shared across validate() calls.
+                    else copy.deepcopy(default.default)
                 )
 
             return new

test_schema.py

@@ -419,6 +419,17 @@ def test_dict_optional_defaults():
         Optional(And(str, Use(int)), default=7)
 
 
+def test_dict_optional_mutable_default_not_shared():
+    # A mutable default (e.g. [] or {}) must not be shared across validate()
+    # calls: mutating one result must not leak into later ones. See GH-352.
+    s = Schema({Optional("items", default=[]): list})
+    a = s.validate({})
+    a["items"].append(1)
+    b = s.validate({})
+    assert b["items"] == []
+    assert a["items"] is not b["items"]
+
+
 def test_dict_subtypes():
     d = defaultdict(int, key=1)
     v = Schema({"key": 1}).validate(d)