Fix save_hyperparameters not crashing on dataclass with init=False

src/lightning/pytorch/CHANGELOG.md

@@ -41,6 +41,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fix `save_last` behavior in the absence of validation ([#20960](https://github.com/Lightning-AI/pytorch-lightning/pull/20960))
 
 
+- Fixed `save_hyperparameters` crashing with `dataclasses` using `init=False` ([#21051](https://github.com/Lightning-AI/pytorch-lightning/pull/21051))
+
+
 ---
 
 ## [2.5.2] - 2025-06-20

src/lightning/pytorch/utilities/parsing.py

@@ -167,7 +167,14 @@ def save_hyperparameters(
     if given_hparams is not None:
         init_args = given_hparams
     elif is_dataclass(obj):
-        init_args = {f.name: getattr(obj, f.name) for f in fields(obj)}
+        obj_fields = fields(obj)
+        init_args = {f.name: getattr(obj, f.name) for f in obj_fields if f.init}
+        if any(not f.init for f in obj_fields):
+            rank_zero_warn(
+                "Detected a dataclass with fields with `init=False`. This is not supported by `save_hyperparameters`"
+                " and will not save those fields in `self.hparams`. Consider removing `init=False` and just"
+                " re-initialize the attributes in the `__post_init__` method of the dataclass."
+            )
     else:
         init_args = {}
 

tests/tests_pytorch/models/test_hparams.py

@@ -17,7 +17,7 @@
 import pickle
 import sys
 from argparse import Namespace
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from enum import Enum
 from unittest import mock
 
@@ -881,6 +881,32 @@ def test_dataclass_lightning_module(tmp_path):
     assert model.hparams == {"mandatory": 33, "optional": "cocofruit"}
 
 
+def test_dataclass_with_init_false_fields():
+    """Test that save_hyperparameters() filters out fields with init=False and issues a warning."""
+
+    @dataclass
+    class DataClassWithInitFalseFieldsModel(BoringModel):
+        mandatory: int
+        optional: str = "optional"
+        non_init_field: int = field(default=999, init=False)
+        another_non_init: str = field(default="not_in_init", init=False)
+
+        def __post_init__(self):
+            super().__init__()
+            self.save_hyperparameters()
+
+    with pytest.warns(UserWarning, match="Detected a dataclass with fields with `init=False`"):
+        model = DataClassWithInitFalseFieldsModel(33, optional="cocofruit")
+
+    expected_hparams = {"mandatory": 33, "optional": "cocofruit"}
+    assert model.hparams == expected_hparams
+
+    assert model.non_init_field == 999
+    assert model.another_non_init == "not_in_init"
+    assert "non_init_field" not in model.hparams
+    assert "another_non_init" not in model.hparams
+
+
 class NoHparamsModel(BoringModel):
     """Tests a model without hparams."""