Sort column headers for csv logger (#19159)

Co-authored-by: awaelchli <[email protected]>

src/lightning/fabric/CHANGELOG.md

@@ -26,6 +26,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Changed the `TransformerEnginePrecision(dtype=)` argument to `weights_dtype` and made it required ([#19082](https://github.com/Lightning-AI/lightning/pull/19082))
 
 
+- The columns in the `metrics.csv` file produced by `CSVLogger` are now sorted alphabetically ([#19159](https://github.com/Lightning-AI/lightning/pull/19159))
+
+
 ### Deprecated
 
 -

src/lightning/fabric/loggers/csv_logs.py

@@ -253,6 +253,7 @@ def _record_new_keys(self) -> Set[str]:
         current_keys = set().union(*self.metrics)
         new_keys = current_keys - set(self.metrics_keys)
         self.metrics_keys.extend(new_keys)
+        self.metrics_keys.sort()
         return new_keys
 
     def _rewrite_with_new_header(self, fieldnames: List[str]) -> None:

src/lightning/pytorch/CHANGELOG.md

@@ -41,6 +41,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Changed the `TransformerEnginePrecision(dtype=)` argument to `weights_dtype` and made it required ([#19082](https://github.com/Lightning-AI/lightning/pull/19082))
 
 
+- The columns in the `metrics.csv` file produced by `CSVLogger` are now sorted alphabetically ([#19159](https://github.com/Lightning-AI/lightning/pull/19159))
+
+
 ### Deprecated
 
 - Deprecated all precision plugin classes under `lightning.pytorch.plugins` with the suffix `Plugin` in the name ([#18840](https://github.com/Lightning-AI/lightning/pull/18840))

tests/tests_fabric/loggers/test_csv.py

@@ -183,3 +183,21 @@ def test_rewrite_with_new_header(tmp_path):
         assert header == new_columns
         logs = file.readline().strip().split(",")
         assert logs == ["0", "1", "22", ""]
+
+
+def test_log_metrics_column_order_sorted(tmp_path):
+    """Test that the columns in the output metrics file are sorted by name."""
+    logger = CSVLogger(tmp_path)
+    logger.log_metrics({"c": 0.1})
+    logger.log_metrics({"c": 0.2})
+    logger.log_metrics({"b": 0.3})
+    logger.log_metrics({"a": 0.4})
+    logger.save()
+    logger.log_metrics({"d": 0.5})
+    logger.save()
+
+    path_csv = os.path.join(logger.log_dir, _ExperimentWriter.NAME_METRICS_FILE)
+    with open(path_csv) as fp:
+        lines = fp.readlines()
+
+    assert lines[0].strip() == "a,b,c,d,step"