BUG: fix a crash when retrieving rows from a multiple-index table

astropy/table/index.py

@@ -30,14 +30,19 @@
     c[1:2] -> reference
     array.view(Column) -> no indices
 """
+from __future__ import annotations
 
 from copy import deepcopy
+from typing import TYPE_CHECKING
 
 import numpy as np
 
 from .bst import MaxValue, MinValue
 from .sorted_array import SortedArray
 
+if TYPE_CHECKING:
+    from collections.abc import Iterable
+
 
 class QueryError(ValueError):
     """
@@ -827,21 +832,27 @@ def __init__(self, table):
         self.table = table
         self.indices = table.indices
 
-    def _get_rows(self, item):
+    def _get_rows(
+        self,
+        item: int | Iterable[int] | slice | tuple[str, slice],
+    ) -> list[np.int64]:
         """
         Retrieve Table rows indexes by value slice.
         """
         if len(self.indices) == 0:
             raise ValueError("Can only use TableLoc for a table with indices")
 
-        if isinstance(item, tuple):
+        if (
+            isinstance(item, tuple)
+            and len(item) == 2
+            and isinstance(item[0], str)
+            and isinstance(item[1], slice)
+        ):
             key, item = item
         else:
             key = self.table.primary_key
 
         index = self.indices[key]
-        if len(index.columns) > 1:
-            raise ValueError("Cannot use .loc on multi-column indices")
 
         if isinstance(item, slice):
             # None signifies no upper/lower bound
@@ -854,7 +865,7 @@ def _get_rows(self, item):
             # item should be a list or ndarray of values
             rows = []
             for key in item:
-                p = index.find((key,))
+                p = index.find(key if isinstance(key, tuple) else (key,))
                 if len(p) == 0:
                     raise KeyError(f"No matches found for key {key}")
                 else:

astropy/table/table.py

@@ -1023,8 +1023,7 @@ def indices(self):
     def loc(self):
         """
         Return a TableLoc object that can be used for retrieving
-        rows by index in a given data range. Note that both loc
-        and iloc work only with single-column indices.
+        rows by index in a given data range.
         """
         return TableLoc(self)
 

astropy/table/tests/test_index.py

@@ -443,6 +443,17 @@ def test_table_loc_indices(self, main_col, table_types, engine):
         for i, p in zip(t2, [1, 4, 2]):  # same order as input list
             assert i == p - 1
 
+    def test_multiple_index_loc(self, main_col, table_types):
+        # see https://github.com/astropy/astropy/issues/13176
+        if isinstance(main_col, Time):
+            pytest.skip()
+        self._setup(main_col, table_types)
+        t = self.t
+        t.add_index(["a", "b"])
+        t2 = t.loc[(1, 4.0)]
+        assert_col_equal(t2["a"], [1])
+        assert_col_equal(t2["b"], [4.0])
+
     def test_invalid_search(self, main_col, table_types, engine):
         # using .loc and .loc_indices with a value not present should raise an exception
         self._setup(main_col, table_types)

docs/changes/table/15826.bugfix.rst

@@ -0,0 +1 @@
+Fix a crash when retrieving rows from a multiple-index table.