🐛 Give a useful error message when input is empty

src/mods4pandas/alto4pandas.py

@@ -3,6 +3,7 @@ import contextlib
 import csv
 import os
 import sqlite3
+import sys
 import warnings
 from operator import attrgetter
 from typing import List
@@ -19,6 +20,7 @@ from .lib import (
     insert_into_db,
     ns,
     sorted_groupby,
+    sqlite3_table_exists,
 )
 
 
@@ -239,6 +241,11 @@ def process(alto_files: List[str], output_file: str):
 
                 traceback.print_exc()
 
+    # Check if table exists
+    if not sqlite3_table_exists(con, "alto_info"):
+        logger.error("Table alto_info does not exist, empty input?")
+        sys.exit(1)
+
     # Convert the alto_info SQL to a pandas DataFrame
     logger.info("Writing DataFrame to {}".format(output_file))
     convert_db_to_parquet(con, "alto_info", "alto_file", output_file)

src/mods4pandas/lib.py

@@ -469,6 +469,12 @@ def convert_db_to_parquet(con, table, index_col, output_file):
     df.to_parquet(output_file)
 
 
+def sqlite3_table_exists(con, table):
+    """Check if table exists."""
+    cur = con.execute("SELECT 1 FROM pragma_table_info(?) LIMIT 1", (table,))
+    return cur.fetchone() is not None
+
+
 def sqlite3_column_exists(con, table, col):
     """Check if column col exists in table."""
     cur = con.execute(

src/mods4pandas/mods4pandas.py

@@ -3,6 +3,7 @@ import contextlib
 import csv
 import os
 import sqlite3
+import sys
 import warnings
 from operator import attrgetter
 from typing import Dict, List
@@ -21,6 +22,7 @@ from .lib import (
     ns,
     sorted_groupby,
     sqlite3_column_exists,
+    sqlite3_table_exists,
 )
 
 
@@ -653,6 +655,11 @@ def process(
             except Exception:
                 logger.exception("Exception in {}".format(mets_file))
 
+    # Check if table exists
+    if not sqlite3_table_exists(con, "mods_info"):
+        logger.error("Table mods_info does not exist, empty input?")
+        sys.exit(1)
+
     logger.info("Writing DataFrame to {}".format(output_file))
 
     considered_indexes = ("recordInfo_recordIdentifier", "recordIdentifier-zdb")