Nicer logging of list of dicts for hyper parameters (#19963)

* Add support for handling lists of dictionaries in logging
* Apply suggestions from code review
* Update CHANGELOG.md

---------

Co-authored-by: Jirka Borovec <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

src/lightning/fabric/CHANGELOG.md

@@ -6,9 +6,15 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ## [unreleased] - YYYY-MM-DD
 
+### Added
+
+- Added logging support for list of dicts without collapsing to a single key ([#19957](https://github.com/Lightning-AI/pytorch-lightning/issues/19957))
+
+
 ### Removed
 
-Removed legacy supoport for `lightning run model`. Use `fabric run` instead. ([#20588](https://github.com/Lightning-AI/pytorch-lightning/pull/20588))
+- Removed legacy supoport for `lightning run model`. Use `fabric run` instead. ([#20588](https://github.com/Lightning-AI/pytorch-lightning/pull/20588))
+
 
 ## [2.5.0] - 2024-12-19
 

src/lightning/fabric/utilities/logger.py

@@ -91,6 +91,8 @@ 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] = {}
@@ -103,6 +105,10 @@ 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

@@ -64,6 +64,12 @@ 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 params == {"dl/0/a": 1, "dl/0/c": 3, "dl/1/b": 2, "dl/1/d": 5, "l": [1, 2, 3, 4]}
+
     # Test flattening of argparse Namespace
     params = Namespace(a=1, b=2)
     wrapping_dict = {"params": params}