Count data rows when validating custom indices

README.md

@@ -153,6 +153,8 @@ To suppress row indices for all types of data, pass `showindex="never"`
 or `showindex=False`. To add a custom row index column, pass
 `showindex=rowIDs`, where `rowIDs` is some iterable:
 
+Custom indices apply to data rows; separating lines do not consume index values.
+
 ```pycon
 >>> print(tabulate([["F",24],["M",19]], showindex="always"))
 -  -  --

tabulate/__init__.py

@@ -1401,12 +1401,12 @@ def _prepend_row_index(rows, index):
     """Add a left-most index column."""
     if index is None or index is False:
         return rows
-    if isinstance(index, Sized) and len(index) != len(rows):
+    sans_rows, separating_lines = _remove_separating_lines(rows)
+    if isinstance(index, Sized) and len(index) != len(sans_rows):
         raise ValueError(
             "index must be as long as the number of data rows: "
-            f"len(index)={len(index)} len(rows)={len(rows)}"
+            f"len(index)={len(index)} len(rows)={len(sans_rows)}"
         )
-    sans_rows, separating_lines = _remove_separating_lines(rows)
     new_rows = []
     index_iter = iter(index)
     for row in sans_rows:
@@ -1599,7 +1599,7 @@ def _normalize_tabular_data(tabular_data, headers, showindex="default"):
         rows = _prepend_row_index(rows, showindex)
     elif showindex == "always" or (_bool(showindex) and not showindex_is_a_str):
         if index is None:
-            index = list(range(len(rows)))
+            index = list(range(sum(not _is_separating_line(row) for row in rows)))
         rows = _prepend_row_index(rows, index)
     elif showindex == "never" or (not _bool(showindex) and not showindex_is_a_str):
         pass

test/test_separator_index.py

@@ -0,0 +1,20 @@
+import pytest
+
+from tabulate import SEPARATING_LINE, tabulate
+
+
+@pytest.mark.parametrize("indices", [["A", "B"], ("A", "B"), iter(["A", "B"])])
+def test_custom_index_skips_separator_rows(indices):
+    table = [["apple"], SEPARATING_LINE, ["pear"]]
+    assert tabulate(table, showindex=indices, tablefmt="plain") == "A  apple\n\nB  pear"
+
+
+def test_automatic_index_skips_separator_rows():
+    table = [["apple"], SEPARATING_LINE, ["pear"]]
+    assert tabulate(table, showindex=True, tablefmt="plain") == "0  apple\n\n1  pear"
+
+
+def test_custom_index_length_counts_only_data_rows():
+    table = [["apple"], SEPARATING_LINE, ["pear"]]
+    with pytest.raises(ValueError, match="len\\(rows\\)=2"):
+        tabulate(table, showindex=["A"], tablefmt="plain")