Handle post_load methods that append to data (#2797)

Author: sloria

CHANGELOG.rst

@@ -14,6 +14,9 @@ Bug fixes:
 
 - Respect ``data_key`` when schema validators raise a `ValidationError <marshmallow.exceptions.ValidationError>` 
   with a ``field_name`` argument (:issue:`2170`). Thanks :user:`matejsp` for reporting.
+- Correctly handle multiple `@post_load <marshmallow.post_load>` methods where one method appends to
+  the data and another passes ``pass_original=True`` (:issue:`1755`).
+  Thanks :user:`ghostwheel42` for reporting.
 
 Documentation:
 

src/marshmallow/schema.py

@@ -15,6 +15,7 @@
 from abc import ABCMeta
 from collections import OrderedDict, defaultdict
 from collections.abc import Mapping
+from itertools import zip_longest
 
 from marshmallow import base, class_registry, types
 from marshmallow import fields as ma_fields
@@ -1291,7 +1292,7 @@ def _invoke_processors(
                 if pass_original:
                     data = [
                         processor(item, original, many=many, **kwargs)
-                        for item, original in zip(data, original_data)
+                        for item, original in zip_longest(data, original_data)
                     ]
                 else:
                     data = [processor(item, many=many, **kwargs) for item in data]

tests/test_decorators.py

@@ -925,3 +925,26 @@ class ExampleSchema(Schema):
 
     schema = ExampleSchema()
     assert schema.load(data) == data
+
+
+# https://github.com/marshmallow-code/marshmallow/issues/1755
+def test_post_load_method_that_appends_to_data():
+    class MySchema(Schema):
+        foo = fields.Int()
+
+        @post_load(pass_many=True)
+        def append_to_data(self, data, **kwargs):
+            data.append({"foo": 42})
+            return data
+
+        @post_load(pass_many=False, pass_original=True)
+        def noop(self, data, original_data, **kwargs):
+            if original_data is None:  # added item
+                assert data == {"foo": 42}
+            else:
+                assert original_data == {"foo": 24}
+                assert data == {"foo": 24}
+            return data
+
+    schema = MySchema(many=True)
+    assert schema.load([{"foo": 24}]) == [{"foo": 24}, {"foo": 42}]