fix bugged singleton implementation

Signed-off-by: Maciej Obuchowski <[email protected]>

tests/utils/test_singleton.py

@@ -23,16 +23,43 @@ class A(metaclass=Singleton):
     pass
 
 
+class Counter(metaclass=Singleton):
+    """Singleton class that counts how much __init__ and count was called."""
+
+    counter = 0
+
+    def __init__(self):
+        self.counter += 1
+
+    def count(self):
+        self.counter += 1
+
+
 def test_singleton_refers_to_same_instance():
     a, b = A(), A()
-    assert a == b
+    assert a is b
 
 
-def test_singleton_out_of_context_does_refer_to_same_instance():
+def test_singleton_after_out_of_context_does_refer_to_same_instance():
+    # check if setting something on singleton is preserved after instance goes out of context
     def x():
         a = A()
         a.a = "a"
 
     x()
     b = A()
     assert b.a == "a"
+
+
+def test_singleton_does_not_call_init_second_time():
+    # first creation of Counter, check if __init__ is called
+    c = Counter()
+    assert c.counter == 1
+
+    # check if "new instance" calls __init__ - it shouldn't
+    d = Counter()
+    assert c.counter == 1
+
+    # check if incrementing "new instance" increments counter on previous one
+    d.count()
+    assert c.counter == 2