Bug fix (GH pandas-dev#23078)

doc/source/whatsnew/v0.24.0.rst

@@ -1210,6 +1210,7 @@ Datetimelike
 - Bug in :class:`DatetimeIndex` where calling ``np.array(dtindex, dtype=object)`` would incorrectly return an array of ``long`` objects (:issue:`23524`)
 - Bug in :class:`Index` where passing a timezone-aware :class:`DatetimeIndex` and `dtype=object` would incorrectly raise a ``ValueError`` (:issue:`23524`)
 - Bug in :class:`Index` where calling ``np.array(dtindex, dtype=object)`` on a timezone-naive :class:`DatetimeIndex` would return an array of ``datetime`` objects instead of :class:`Timestamp` objects, potentially losing nanosecond portions of the timestamps (:issue:`23524`)
+- Bug in :class:`PeriodIndex` when comparing indexes of different lengths, ValueError is not raised (:issue:`23078`)
 
 Timedelta
 ^^^^^^^^^

pandas/core/arrays/period.py

@@ -73,6 +73,9 @@ def wrapper(self, other):
                 msg = DIFFERENT_FREQ_INDEX.format(self.freqstr, other.freqstr)
                 raise IncompatibleFrequency(msg)
 
+            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')