don't put checkpoint files inside namespace directories.

src/wikiq/__init__.py

@@ -322,7 +322,7 @@ class WikiqParser:
         """Open checkpoint file for writing. Keeps file open for performance."""
         if not self.output_parquet or output_file == sys.stdout.buffer:
             return
-        checkpoint_path = get_checkpoint_path(output_file)
+        checkpoint_path = get_checkpoint_path(output_file, self.partition_namespaces)
         Path(checkpoint_path).parent.mkdir(parents=True, exist_ok=True)
         self.checkpoint_file = open(checkpoint_path, 'w')
         print(f"Checkpoint file opened: {checkpoint_path}", file=sys.stderr)

src/wikiq/resume.py

@@ -14,12 +14,24 @@ import sys
 import pyarrow.parquet as pq
 
 
-def get_checkpoint_path(output_file):
+def get_checkpoint_path(output_file, partition_namespaces=False):
-    """Get the path to the checkpoint file for a given output file."""
+    """Get the path to the checkpoint file for a given output file.
+
+    For partitioned output, the checkpoint is placed outside the partition directory
+    to avoid pyarrow trying to read it as a parquet file. The filename includes
+    the output filename to keep it unique per input file (for parallel jobs).
+    """
+    if partition_namespaces:
+        # output_file is like partition_dir/output.parquet
+        # checkpoint should be at parent level: parent/output.parquet.checkpoint
+        partition_dir = os.path.dirname(output_file)
+        output_filename = os.path.basename(output_file)
+        parent_dir = os.path.dirname(partition_dir)
+        return os.path.join(parent_dir, output_filename + ".checkpoint")
     return str(output_file) + ".checkpoint"
 
 
-def read_checkpoint(output_file):
+def read_checkpoint(output_file, partition_namespaces=False):
     """
     Read resume point from checkpoint file if it exists.
 
@@ -31,7 +43,7 @@ def read_checkpoint(output_file):
         For single files: A tuple (pageid, revid), or None if not found.
         For partitioned: A dict mapping namespace -> (pageid, revid), or None.
     """
-    checkpoint_path = get_checkpoint_path(output_file)
+    checkpoint_path = get_checkpoint_path(output_file, partition_namespaces)
     if not os.path.exists(checkpoint_path):
         return None
 
@@ -78,7 +90,7 @@ def get_resume_point(output_file, partition_namespaces=False):
                         or None if no partitions exist.
     """
     # First try checkpoint file (fast)
-    checkpoint_result = read_checkpoint(output_file)
+    checkpoint_result = read_checkpoint(output_file, partition_namespaces)
     if checkpoint_result is not None:
         print(f"Resume point found in checkpoint file", file=sys.stderr)
         return checkpoint_result