lovasoa: Thank you for your review @sloria ! I'm merging this.
diff --git a/marshmallow_dataclass/__init__.py b/marshmallow_dataclass/__init__.py
index 67e7d77..403da98 100644
--- a/marshmallow_dataclass/__init__.py
+++ b/marshmallow_dataclass/__init__.py
@@ -34,10 +34,9 @@ Full example::
       })
       Schema: ClassVar[Type[Schema]] = Schema # For the type checker
 """
-import dataclasses
 import inspect
-from functools import lru_cache
 from enum import EnumMeta
+from functools import lru_cache
 from typing import (
     overload,
     Dict,
@@ -54,6 +53,7 @@ from typing import (
     Set,
 )
 
+import dataclasses
 import marshmallow
 import typing_inspect
 
@@ -457,12 +457,17 @@ def _base_schema(
     Base schema factory that creates a schema for `clazz` derived either from `base_schema`
     or `BaseSchema`
     """
+
     # Remove `type: ignore` when mypy handles dynamic base classes
     # https://github.com/python/mypy/issues/2813
     class BaseSchema(base_schema or marshmallow.Schema):  # type: ignore
-        @marshmallow.post_load
-        def make_data_class(self, data, **_):
-            return clazz(**data)
+        def load(self, data: Mapping, *, many: bool = None, **kwargs):
+            all_loaded = super().load(data, many=many, **kwargs)
+            many = self.many if many is None else bool(many)
+            if many:
+                return [clazz(**loaded) for loaded in all_loaded]
+            else:
+                return clazz(**all_loaded)
 
     return BaseSchema
 

Behavior of marshmallow.post_load is dependent on function name
I discovered a discrepancy with how the `post_load` decorator behaves within a marshmallow dataclass. In [Schema._invoke_processors](https://github.com/marshmallow-code/marshmallow/blob/76196abf35ff9ec58f3dc2377ea7a8a9bf23712a/src/marshmallow/schema.py#L1192) the post loads `make_data_class` processor runs before or after the post_load defined in the dataclass based on whichever comes first alphabetically. Some example below illustrate this: 

```python
import marshmallow
import marshmallow_dataclass

@marshmallow_dataclass.dataclass
class Person:
	name: str

	@marshmallow.post_load
	def a(schema, data, **kwargs):
		data.name = data.name.capitalize()  # works if: data['name'] = data['name'].capitalize()
		return data

Person.Schema().load({'name': 'matt'})
>> AttributeError: 'dict' object has no attribute 'name'
```

```python
import marshmallow
import marshmallow_dataclass

@marshmallow_dataclass.dataclass
class Person:
	name: str

	@marshmallow.post_load
	def z(schema, data, **kwargs):
		data.name = data.name.capitalize()
		return data

Person.Schema().load({'name': 'matt'})
>> Person(name='Matt')
```

The post_load in the first example does not create an instance of the dataclass itself before being invoked and feels more so like a `pre_load` because of this. The validation checks still seem to work properly, but it just feels a little weird to work with. I know the documentation does not include anything about pre/post loading, but I figured I'd document this in case anyone else runs into the same issue.
diff --git a/tests/test_post_load.py b/tests/test_post_load.py
new file mode 100644
index 0000000..2e53c2c
--- /dev/null
+++ b/tests/test_post_load.py
@@ -0,0 +1,44 @@
+import unittest
+
+import marshmallow
+
+import marshmallow_dataclass
+
+
+# Regression test for https://github.com/lovasoa/marshmallow_dataclass/issues/75
+class TestPostLoad(unittest.TestCase):
+    @marshmallow_dataclass.dataclass
+    class Named:
+        first_name: str
+        last_name: str
+
+        @marshmallow.post_load
+        def a(self, data, **_kwargs):
+            data["first_name"] = data["first_name"].capitalize()
+            return data
+
+        @marshmallow.post_load
+        def z(self, data, **_kwargs):
+            data["last_name"] = data["last_name"].capitalize()
+            return data
+
+    def test_post_load_method_naming_does_not_affect_data(self):
+        actual = self.Named.Schema().load(
+            {"first_name": "matt", "last_name": "groening"}
+        )
+        expected = self.Named(first_name="Matt", last_name="Groening")
+        self.assertEqual(actual, expected)
+
+    def test_load_many(self):
+        actual = self.Named.Schema().load(
+            [
+                {"first_name": "matt", "last_name": "groening"},
+                {"first_name": "bart", "last_name": "simpson"},
+            ],
+            many=True,
+        )
+        expected = [
+            self.Named(first_name="Matt", last_name="Groening"),
+            self.Named(first_name="Bart", last_name="Simpson"),
+        ]
+        self.assertEqual(actual, expected)

pip install -e .[dev]
parse_log_pytest
requirements.txt
pytest pytest-mypy-plugins
apt-get update apt-get install -y gcc
3.9
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
alabaster==0.7.16
aspy.yaml==1.3.0
attrs==25.3.0
babel==2.17.0
certifi==2025.1.31
cfgv==3.4.0
charset-normalizer==3.4.1
decorator==5.2.1
distlib==0.3.9
docutils==0.21.2
exceptiongroup==1.2.2
filelock==3.18.0
identify==2.6.9
idna==3.10
imagesize==1.4.1
importlib_metadata==8.6.1
iniconfig==2.1.0
Jinja2==3.1.6
jsonschema==4.23.0
jsonschema-specifications==2024.10.1
MarkupSafe==3.0.2
marshmallow==3.26.1
marshmallow-enum==1.5.1
marshmallow-union==0.1.15.post1
-e git+https://github.com/lovasoa/marshmallow_dataclass.git@69caec49402adb07149ff87700ef52eeeeba0066#egg=marshmallow_dataclass
mypy==1.15.0
mypy-extensions==1.0.0
nodeenv==1.9.1
packaging==24.2
platformdirs==4.3.7
pluggy==1.5.0
pre-commit==1.21.0
Pygments==2.19.1
pytest==8.3.5
pytest-mypy-plugins==3.2.0
PyYAML==6.0.2
referencing==0.36.2
regex==2024.11.6
requests==2.32.3
rpds-py==0.24.0
six==1.17.0
snowballstemmer==2.2.0
Sphinx==7.4.7
sphinxcontrib-applehelp==2.0.0
sphinxcontrib-devhelp==2.0.0
sphinxcontrib-htmlhelp==2.1.0
sphinxcontrib-jsmath==1.0.1
sphinxcontrib-qthelp==2.0.0
sphinxcontrib-serializinghtml==2.0.0
toml==0.10.2
tomli==2.2.1
tomlkit==0.13.2
typing-inspect==0.9.0
typing_extensions==4.13.0
urllib3==2.3.0
virtualenv==20.29.3
zipp==3.21.0

name: marshmallow_dataclass
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=3.0.16=h5eee18b_0
  - pip=25.0=py39h06a4308_0
  - python=3.9.21=he870216_1
  - readline=8.2=h5eee18b_0
  - setuptools=75.8.0=py39h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - tzdata=2025a=h04d1e81_0
  - wheel=0.45.1=py39h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - alabaster==0.7.16
      - aspy-yaml==1.3.0
      - attrs==25.3.0
      - babel==2.17.0
      - certifi==2025.1.31
      - cfgv==3.4.0
      - charset-normalizer==3.4.1
      - decorator==5.2.1
      - distlib==0.3.9
      - docutils==0.21.2
      - exceptiongroup==1.2.2
      - filelock==3.18.0
      - identify==2.6.9
      - idna==3.10
      - imagesize==1.4.1
      - importlib-metadata==8.6.1
      - iniconfig==2.1.0
      - jinja2==3.1.6
      - jsonschema==4.23.0
      - jsonschema-specifications==2024.10.1
      - markupsafe==3.0.2
      - marshmallow==3.26.1
      - marshmallow-dataclass==7.5.0
      - marshmallow-enum==1.5.1
      - marshmallow-union==0.1.15.post1
      - mypy==1.15.0
      - mypy-extensions==1.0.0
      - nodeenv==1.9.1
      - packaging==24.2
      - platformdirs==4.3.7
      - pluggy==1.5.0
      - pre-commit==1.21.0
      - pygments==2.19.1
      - pytest==8.3.5
      - pytest-mypy-plugins==3.2.0
      - pyyaml==6.0.2
      - referencing==0.36.2
      - regex==2024.11.6
      - requests==2.32.3
      - rpds-py==0.24.0
      - six==1.17.0
      - snowballstemmer==2.2.0
      - sphinx==7.4.7
      - sphinxcontrib-applehelp==2.0.0
      - sphinxcontrib-devhelp==2.0.0
      - sphinxcontrib-htmlhelp==2.1.0
      - sphinxcontrib-jsmath==1.0.1
      - sphinxcontrib-qthelp==2.0.0
      - sphinxcontrib-serializinghtml==2.0.0
      - toml==0.10.2
      - tomli==2.2.1
      - tomlkit==0.13.2
      - typing-extensions==4.13.0
      - typing-inspect==0.9.0
      - urllib3==2.3.0
      - virtualenv==20.29.3
      - zipp==3.21.0
prefix: /opt/conda/envs/marshmallow_dataclass

tests/test_post_load.py::TestPostLoad::test_load_many tests/test_post_load.py::TestPostLoad::test_post_load_method_naming_does_not_affect_data
marshmallow_dataclass/__init__.py
marshmallow_dataclass/__init__.py