refactor num_threads

HISTORY.rst

@@ -12,6 +12,7 @@ Bug tracker at https://github.com/giampaolo/psutil/issues
   - Process.ppid() is 20% faster
   - Process.status() is 28% faster
   - Process.name() is 25% faster
+  - Process.num_threads is 20% faster on Python 3
 
 **Bug fixes**
 

psutil/_pslinux.py

@@ -967,6 +967,10 @@ def _parse_stat_file(self):
         fields_after_name = data[rpar + 2:].split()
         return [name] + fields_after_name
 
+    def _read_status_file(self):
+        with open_binary("%s/%s/status" % (self._procfs_path, self.pid)) as f:
+            return f.read()
+
     @wrap_exceptions
     def name(self):
         name = self._parse_stat_file()[0]
@@ -1208,12 +1212,12 @@ def num_ctx_switches(self):
                 "probably older than 2.6.23" % self.pid)
 
     @wrap_exceptions
-    def num_threads(self):
-        with open_binary("%s/%s/status" % (self._procfs_path, self.pid)) as f:
-            for line in f:
-                if line.startswith(b"Threads:"):
-                    return int(line.split()[1])
-            raise NotImplementedError("line not found")
+    def num_threads(self, _num_threads_re=re.compile(b'Threads:\t(\d+)')):
+        # Note: on Python 3 using a re is faster than iterating over file
+        # line by line. On Python 2 is the exact opposite, and iterating
+        # over a file on Python 3 is slower than on Python 2.
+        data = self._read_status_file()
+        return int(_num_threads_re.findall(data)[0])
 
     @wrap_exceptions
     def threads(self):

psutil/tests/test_linux.py

@@ -804,6 +804,12 @@ def test_compare_stat_and_status_files(self):
                         elif line.startswith('PPid:'):
                             ppid = int(line.split()[1])
                             self.assertEqual(p.ppid(), ppid)
+                        elif line.startswith('Threads:'):
+                            # num_threads() is determined via 'status' file
+                            # but we use a re internally, so let's check it
+                            # also.
+                            num_threads = int(line.split()[1])
+                            self.assertEqual(p.num_threads(), num_threads)
 
     def test_memory_maps(self):
         src = textwrap.dedent("""
@@ -924,13 +930,6 @@ def test_num_ctx_switches_mocked(self):
                 psutil._pslinux.Process(os.getpid()).num_ctx_switches)
             assert m.called
 
-    def test_num_threads_mocked(self):
-        with mock.patch('psutil._pslinux.open', create=True) as m:
-            self.assertRaises(
-                NotImplementedError,
-                psutil._pslinux.Process(os.getpid()).num_threads)
-            assert m.called
-
     def test_uids_mocked(self):
         with mock.patch('psutil._pslinux.open', create=True) as m:
             self.assertRaises(