Return a deepcopy of the result in cachedResult

Author: Maxcode123

src/unittest_extensions/case.py

@@ -81,10 +81,15 @@ def cachedResult(self) -> Any:
         subject without executing it multiple times.
 
         Raises `unittest_extensions.TestError` if subject has not been called.
+
+        The returned object is a copy of the result. Thus, the result cannot be
+        mutated by mutating the returned object of this method.
         """
         if not hasattr(self, "_subject_result"):
             raise TestError("Cannot call 'cachedResult' before calling 'result'")
-        return self._subject_result
+        # NOTE: deepcopy keeps a reference of the copied object. This can cause
+        # issues with memory.
+        return deepcopy(self._subject_result)
 
     def assertResult(self, value):
         """

src/unittest_extensions/tests/test_use_case.py

@@ -182,3 +182,14 @@ def subject(self, lst):
     def test_mutable_kwargs(self):
         self.subjectKwargs()["lst"].append(3)
         self.assertDictEqual(self._subjectKwargs, {"lst": [1, 2]})
+
+
+class TestCachedResult(TestCase):
+    def subject(self, lst):
+        return lst
+
+    @args({"lst": [1, 2]})
+    def test_mutate_cached_result(self):
+        self.assertResult([1, 2])
+        self.cachedResult().append(3)
+        self.assertListEqual(self.cachedResult(), [1, 2])