fixes #26622: pd.MultiIndex.isin should fail when input args are too short

doc/source/whatsnew/v0.25.0.rst

@@ -608,6 +608,7 @@ MultiIndex
 ^^^^^^^^^^
 
 - Bug in which incorrect exception raised by :class:`Timedelta` when testing the membership of :class:`MultiIndex` (:issue:`24570`)
+- :meth:`MultiIndex.isin` now raises ``ValueError`` when items in values are not the same length as the number of levels in the MultiIndex (:issue:`26622`)
 -
 
 I/O

pandas/core/indexes/multi.py

@@ -3175,6 +3175,14 @@ def _wrap_joined_index(self, joined, other):
     @Appender(Index.isin.__doc__)
     def isin(self, values, level=None):
         if level is None:
+            # validate value length
+            nlvl = len(self.levels)
+            for val in values:
+                if len(val) != nlvl:
+                    raise ValueError('Length of each element in values must '
+                                     'match number of levels in MultiIndex. '
+                                     'len({}) != {}'.format(val, nlvl))
+
             values = MultiIndex.from_tuples(values,
                                             names=self.names).values
             return algos.isin(self.values, values)

pandas/tests/indexes/multi/test_contains.py

@@ -1,3 +1,5 @@
+import re
+
 import numpy as np
 import pytest
 
@@ -58,6 +60,25 @@ def test_isin():
     assert result.dtype == np.bool_
 
 
+@pytest.mark.parametrize('values, expected_msg',
+                         [([('foo',), ('bar', 3), ('quux',)],
+                           "len(('foo',)) != 2"),
+                          ([('foo', 2), ('bar', 3, 2), ('quux', 4)],
+                           "len(('bar', 3, 2)) != 2")])
+def test_isin_bad_values_raises(values, expected_msg):
+    idx = MultiIndex.from_arrays([
+        ['qux', 'baz', 'foo', 'bar'],
+        np.arange(4)
+    ])
+
+    msg = re.escape('Length of each element in values must '
+                    'match number of levels in MultiIndex. '
+                    + expected_msg)
+
+    with pytest.raises(ValueError, match=msg):
+        idx.isin(values)
+
+
 @pytest.mark.skipif(PYPY, reason="tuples cmp recursively on PyPy")
 def test_isin_nan_not_pypy():
     idx = MultiIndex.from_arrays([['foo', 'bar'], [1.0, np.nan]])