BUG: .isin on datetimelike indexes do not validate input of level parameter

doc/source/whatsnew/v0.25.0.rst

@@ -540,6 +540,7 @@ Datetimelike
 - Bug in :func:`to_datetime` which does not replace the invalid argument with ``NaT`` when error is set to coerce (:issue:`26122`)
 - Bug in adding :class:`DateOffset` with nonzero month to :class:`DatetimeIndex` would raise ``ValueError`` (:issue:`26258`)
 - Bug in :func:`to_datetime` which raises unhandled ``OverflowError`` when called with mix of invalid dates and ``NaN`` values with ``format='%Y%m%d'`` and ``error='coerce'`` (:issue:`25512`)
+- Bug in :meth:`isin` for datetimelike indexes; :class:`DatetimeIndex`, :class:`TimedeltaIndex` and :class:`PeriodIndex` where the ``levels`` parameter was ignored. (:issue:`26675`)
 - Bug in :func:`to_datetime` which raises ``TypeError`` for ``format='%Y%m%d'`` when called for invalid integer dates with length >= 6 digits with ``errors='ignore'``
 - Bug when comparing a :class:`PeriodIndex` against a zero-dimensional numpy array (:issue:`26689`)
 

pandas/core/indexes/datetimelike.py

@@ -522,6 +522,9 @@ def isin(self, values, level=None):
         -------
         is_contained : ndarray (boolean dtype)
         """
+        if level is not None:
+            self._validate_index_level(level)
+
         if not isinstance(values, type(self)):
             try:
                 values = type(self)(values)

pandas/tests/indexes/test_base.py

@@ -1791,22 +1791,23 @@ def test_isin_level_kwarg(self, level, index):
         tm.assert_numpy_array_equal(expected,
                                     index.isin(values, level='foobar'))
 
-    @pytest.mark.parametrize("level", [1, 10, -2])
-    @pytest.mark.parametrize("index", [
-        Index(['qux', 'baz', 'foo', 'bar']),
-        # Float64Index overrides isin, so must be checked separately
-        Float64Index([1.0, 2.0, 3.0, 4.0])])
-    def test_isin_level_kwarg_raises_bad_index(self, level, index):
+    @pytest.mark.parametrize("level", [2, 10, -3])
+    def test_isin_level_kwarg_bad_level_raises(self, level, indices):
+        index = indices
         with pytest.raises(IndexError, match='Too many levels'):
             index.isin([], level=level)
 
-    @pytest.mark.parametrize("level", [1.0, 'foobar', 'xyzzy', np.nan])
-    @pytest.mark.parametrize("index", [
-        Index(['qux', 'baz', 'foo', 'bar']),
-        Float64Index([1.0, 2.0, 3.0, 4.0])])
-    def test_isin_level_kwarg_raises_key(self, level, index):
-        with pytest.raises(KeyError, match='must be same as name'):
-            index.isin([], level=level)
+    @pytest.mark.parametrize("label", [1.0, 'foobar', 'xyzzy', np.nan])
+    def test_isin_level_kwarg_bad_label_raises(self, label, indices):
+        index = indices
+        if isinstance(index, MultiIndex):
+            index = index.rename(['foo', 'bar'])
+            msg = "'Level {} not found'"
+        else:
+            index = index.rename('foo')
+            msg = r"'Level {} must be same as name \(foo\)'"
+        with pytest.raises(KeyError, match=msg.format(label)):
+            index.isin([], level=label)
 
     @pytest.mark.parametrize("empty", [[], Series(), np.array([])])
     def test_isin_empty(self, empty):