Fix #1055, fix #1085, fix #1087.

- no longer cache cpu_count() return value in Process.cpu_percent()
- in Process.cpu_percent(), guard against cpu_count() returning None and
  assume 1 instead
- add test cases

HISTORY.rst

@@ -50,7 +50,9 @@
 - 1047_: [Windows] Process username(): memory leak in case exception is thrown.
 - 1048_: [Windows] users()'s host field report an invalid IP address.
 - 1050_: [Windows] Process.memory_maps memory() leaks memory.
-- 1055_: cpu_count() is no longer cached.
+- 1055_: cpu_count() is no longer cached; this is useful on systems such as
+  Linux where CPUs can be disabled at runtime. This also reflects on
+  Process.cpu_percent() which no longer uses the cache.
 - 1058_: fixed Python warnings.
 - 1062_: disk_io_counters() and net_io_counters() raise TypeError if no disks
   or NICs are installed on the system.

psutil/__init__.py

@@ -209,7 +209,6 @@
 POWER_TIME_UNLIMITED = _common.POWER_TIME_UNLIMITED
 POWER_TIME_UNKNOWN = _common.POWER_TIME_UNKNOWN
 _TOTAL_PHYMEM = None
-_NUM_CPUS = None
 _timer = getattr(time, 'monotonic', time.time)
 
 
@@ -1043,9 +1042,7 @@ def cpu_percent(self, interval=None):
         blocking = interval is not None and interval > 0.0
         if interval is not None and interval < 0:
             raise ValueError("interval is not positive (got %r)" % interval)
-        # TODO: rarely cpu_count() may return None, meaning this will
-        # break. It's probably wise to fall back to 1.
-        num_cpus = _NUM_CPUS or cpu_count()
+        num_cpus = cpu_count() or 1
 
         def timer():
             return _timer() * num_cpus
@@ -1645,10 +1642,8 @@ def cpu_count(logical=True):
 
     >>> psutil.cpu_count.cache_clear()
     """
-    global _NUM_CPUS
     if logical:
         ret = _psplatform.cpu_count_logical()
-        _NUM_CPUS = ret
     else:
         ret = _psplatform.cpu_count_physical()
     return ret if ret >= 1 else None

psutil/tests/test_process.py

@@ -246,6 +246,12 @@ def test_cpu_percent(self):
         with self.assertRaises(ValueError):
             p.cpu_percent(interval=-1)
 
+    def test_cpu_percent_numcpus_none(self):
+        # See: https://github.com/giampaolo/psutil/issues/1087
+        with mock.patch('psutil.cpu_count', return_value=None) as m:
+            psutil.Process().cpu_percent()
+            assert m.called
+
     def test_cpu_times(self):
         times = psutil.Process().cpu_times()
         assert (times.user > 0.0) or (times.system > 0.0), times

psutil/tests/test_system.py

@@ -270,6 +270,18 @@ def test_cpu_count(self):
         self.assertGreaterEqual(physical, 1)
         self.assertGreaterEqual(logical, physical)
 
+    def test_cpu_count_none(self):
+        # https://github.com/giampaolo/psutil/issues/1085
+        for val in (-1, 0, None):
+            with mock.patch('psutil._psplatform.cpu_count_logical',
+                            return_value=val) as m:
+                self.assertIsNone(psutil.cpu_count())
+                assert m.called
+            with mock.patch('psutil._psplatform.cpu_count_physical',
+                            return_value=val) as m:
+                self.assertIsNone(psutil.cpu_count(logical=False))
+                assert m.called
+
     def test_cpu_times(self):
         # Check type, value >= 0, str().
         total = 0