tweak sequence generator (#2)

Author: joshuadavidthomas

tests/utils.py

@@ -11,17 +11,18 @@ def __init__(self):
         self._counter = 1
 
     def __new__(cls):
-        with cls._lock:
-            if cls._instance is None:
-                cls._instance = super().__new__(cls)
-                cls._instance._counter = 1
-            return cls._instance
+        if cls._instance is None:
+            with cls._lock:
+                if cls._instance is None:
+                    cls._instance = super().__new__(cls)
+                    cls._instance._counter = 1
+        return cls._instance
 
     def next(self):
         with self._lock:
             current = self._counter
             self._counter += 1
-            return current
+        return current
 
 
 seq = SequenceGenerator()