CSVLogger: change _load_existing_data handling of partly valid CSVs

In case an exception occurrs during reading (e.g. corrupted CSV, or CSV that misses 'step' column), the reader now starts with an empty CSV instead of keeping the succeeded rows from the existent CSV.
C-Achard · Feb 13, 2026 · Jan 13, 2026 · Jan 16, 2026 · Jan 16, 2026 · Jan 16, 2026
commit dfa3c3f0f4c0623a2f98c6247a75dc1e48ffbd74
diff --git a/deeplabcut/pose_estimation_pytorch/runners/logger.py b/deeplabcut/pose_estimation_pytorch/runners/logger.py
@@ -456,22 +456,26 @@ def log_config(self, config: dict = None) -> None:
 
     def _load_existing_data(self) -> None:
         """Loads existing CSV data if the log file exists"""
+        logging.info(f"Loading existing CSV data from {self.log_file}")
         try:
             with open(self.log_file, "r", newline="") as f:
                 reader = csv.DictReader(f)
 
                 # Update logged metrics from header
                 if "step" not in reader.fieldnames:
-                    return  # Invalid format
+                    raise ValueError("Invalid CSV format: missing 'step' column")
 
                 metric_names = [m for m in reader.fieldnames if m != "step"]
                 self._logged_metrics.update(metric_names)
 
                 # Load data rows
+                steps = []
+                metric_store = []
                 for row in reader:
                     try:
                         step = int(row["step"])
                     except (ValueError, KeyError):
+                        logging.warning(f"Invalid step value in row: {row}")
                         continue
 
                     # Convert metric values: empty strings -> None, numeric strings -> float
@@ -486,14 +490,17 @@ def _load_existing_data(self) -> None:
                             except ValueError:
                                 step_metrics[metric] = value
 
-                    self._steps.append(step)
-                    self._metric_store.append(step_metrics)
+                    steps.append(step)
+                    metric_store.append(step_metrics)
 
         except Exception as e:
             logging.warning(
                 f"Failed to load existing CSV data from {self.log_file}: {e}. "
                 "Starting with empty log."
             )
+            return
+        self._steps.extend(steps)
+        self._metric_store.extend(metric_store)
 
     def _prepare_logs(self) -> list[list]:
         """Prepares the data to log as a list of strings"""