Bug fix (GH pandas-dev#23078)

doc/source/whatsnew/v0.24.0.rst

@@ -1366,6 +1366,7 @@ Datetimelike
 - Bug in :class:`DatetimeIndex` where constructing a :class:`DatetimeIndex` from a :class:`Categorical` or :class:`CategoricalIndex` would incorrectly drop timezone information (:issue:`18664`)
 - Bug in :class:`DatetimeIndex` and :class:`TimedeltaIndex` where indexing with ``Ellipsis`` would incorrectly lose the index's ``freq`` attribute (:issue:`21282`)
 - Clarified error message produced when passing an incorrect ``freq`` argument to :class:`DatetimeIndex` with ``NaT`` as the first entry in the passed data (:issue:`11587`)
+- Bug in :class:`PeriodIndex` when comparing indexes of different lengths, ValueError is not raised (:issue:`23078`)
 
 Timedelta
 ^^^^^^^^^

pandas/core/arrays/period.py

@@ -68,6 +68,9 @@ def wrapper(self, other):
         elif isinstance(other, cls):
             self._check_compatible_with(other)
 
+            if other.ndim > 0 and len(self) != len(other):
+                raise ValueError('Lengths must match to compare')
+
             if not_implemented:
                 return NotImplemented
             result = op(other.asi8)

pandas/tests/indexes/period/test_period.py

@@ -561,6 +561,12 @@ def test_insert(self):
             result = period_range('2017Q1', periods=4, freq='Q').insert(1, na)
             tm.assert_index_equal(result, expected)
 
+    def test_comp_op(self):
+        # GH 23078
+        index = period_range('2017', periods=12, freq="A-DEC")
+        with pytest.raises(ValueError, match="Lengths must match"):
+            index <= index[[0]]
+
 
 def test_maybe_convert_timedelta():
     pi = PeriodIndex(['2000', '2001'], freq='D')