BUG: isnull doesn't properly check for inf when requested

doc/source/v0.14.1.txt

@@ -82,3 +82,6 @@ Bug Fixes
   (:issue:`7140`).
 - Bug in ``StringMethods.extract()`` where a single match group Series
   would use the matcher's name instead of the group name (:issue:`7313`).
+- Bug in ``isnull()`` when ``mode.use_inf_as_null == True`` where isnull
+  wouldn't test ``True`` when it encountered an ``inf``/``-inf``
+  (:issue:`7315`).

pandas/src/util.pxd

@@ -76,8 +76,7 @@ cdef inline bint _checknull_old(object val):
     cdef double INF = <double> np.inf
     cdef double NEGINF = -INF
     try:
-        return bool(val is None or val != val and val != INF
-                    and val != NEGINF)
+        return val is None or val != val or val == INF or val == NEGINF
     except ValueError:
         return False
 

pandas/tests/test_series.py

@@ -2941,6 +2941,16 @@ def test_raise_on_info(self):
         with tm.assertRaises(AttributeError):
             s.info()
 
+    def test_isnull_for_inf(self):
+        s = Series(['a', np.inf, np.nan, 1.0])
+        with pd.option_context('mode.use_inf_as_null', True):
+            r = s.isnull()
+            dr = s.dropna()
+        e = Series([False, True, True, False])
+        de = Series(['a', 1.0], index=[0, 3])
+        tm.assert_series_equal(r, e)
+        tm.assert_series_equal(dr, de)
+
 
 # TimeSeries-specific