Tests

tests/test_canonicaljson.py

@@ -13,6 +13,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from unittest.mock import Mock
 
 from math import inf, nan
 
@@ -21,7 +22,7 @@
     encode_pretty_printed_json,
     iterencode_canonical_json,
     iterencode_pretty_printed_json,
-    set_json_library,
+    set_json_library, register_preserialisation_callback,
 )
 
 import unittest
@@ -156,4 +157,32 @@ class C:
             pass
 
         with self.assertRaises(Exception):
-            encode_canonical_json(C())
+            encode_canonical_json(C())
+
+    def test_preserialisation_callback(self) -> None:
+        class C:
+            pass
+
+        register_preserialisation_callback(C, lambda c: "I am a C instance")
+
+        result = encode_canonical_json(C())
+        self.assertEqual(result, b'"I am a C instance"')
+
+    def test_cannot_register_preserialisation_callback_for_object(self) -> None:
+        with self.assertRaises(Exception):
+            register_preserialisation_callback(object, lambda c: "shouldn't be able to do this")
+
+    def test_most_recent_preserialisation_callback_called(self) -> None:
+        class C:
+            pass
+
+        callback1 = Mock(return_value = "callback 1 was called")
+        callback2 = Mock(return_value = "callback 2 was called")
+
+        register_preserialisation_callback(C, callback1)
+        register_preserialisation_callback(C, callback2)
+
+        encode_canonical_json(C())
+
+        callback1.assert_not_called()
+        callback2.assert_called_once()