Nicer logging of list of dicts for hyper parameters

src/lightning/fabric/CHANGELOG.md

@@ -10,6 +10,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 ### Added
 
 - Added sanitization for classes before logging them as hyperparameters ([#19771](https://github.com/Lightning-AI/pytorch-lightning/pull/19771))
+- Added logging support for list of dicts without collapsing to a single key ([#19957](https://github.com/Lightning-AI/pytorch-lightning/issues/19957))
 - Enabled consolidating distributed checkpoints through `fabric consolidate` in the new CLI ([#19560](https://github.com/Lightning-AI/pytorch-lightning/pull/19560))
 - Added the ability to explicitly mark forward methods in Fabric via `_FabricModule.mark_forward_method()` ([#19690](https://github.com/Lightning-AI/pytorch-lightning/pull/19690))
 - Added support for PyTorch 2.3 ([#19708](https://github.com/Lightning-AI/pytorch-lightning/pull/19708))

src/lightning/fabric/utilities/logger.py

@@ -89,6 +89,8 @@ def _flatten_dict(params: MutableMapping[Any, Any], delimiter: str = "/", parent
         {'a/b': 123}
         >>> _flatten_dict({5: {'a': 123}})
         {'5/a': 123}
+        >>> _flatten_dict({"dl": [{"a": 1, "c": 3}, {"b": 2, "d": 5}], "l": [1, 2, 3, 4]})
+        {'dl/0/a': 1, 'dl/0/c': 3, 'dl/1/b': 2, 'dl/1/d': 5, 'l': [1, 2, 3, 4]}
 
     """
     result: Dict[str, Any] = {}
@@ -101,6 +103,10 @@ def _flatten_dict(params: MutableMapping[Any, Any], delimiter: str = "/", parent
 
         if isinstance(v, MutableMapping):
             result = {**result, **_flatten_dict(v, parent_key=new_key, delimiter=delimiter)}
+        # Also handle the case where v is a list of dictionaries
+        elif isinstance(v, list) and all(isinstance(item, MutableMapping) for item in v):
+            for i, item in enumerate(v):
+                result = {**result, **_flatten_dict(item, parent_key=f"{new_key}/{i}", delimiter=delimiter)}
         else:
             result[new_key] = v
     return result

tests/tests_fabric/utilities/test_logger.py

@@ -63,6 +63,17 @@ def test_flatten_dict():
     assert params["c/8"] == "foo"
     assert params["c/9/10"] == "bar"
 
+    # Test list of nested dicts flattening
+    params = {"dl": [{"a": 1, "c": 3}, {"b": 2, "d": 5}], "l": [1, 2, 3, 4]}
+    params = _flatten_dict(params)
+
+    assert "dl" not in params
+    assert params["dl/0/a"] == 1
+    assert params["dl/0/c"] == 3
+    assert params["dl/1/b"] == 2
+    assert params["dl/1/d"] == 5
+    assert params["l"] == [1, 2, 3, 4]
+
     # Test flattening of argparse Namespace
     params = Namespace(a=1, b=2)
     wrapping_dict = {"params": params}