Singleton Patterns

Author: MilovanTomasevic

src/1_patterns/2_CreationalPatterns/4_Singleton/4_monostate.py

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+# all members are static :)
+
+class CEO:
+    __shared_state = {
+        'name': 'Steve',
+        'age': 55
+    }
+
+    def __init__(self):
+        self.__dict__ = self.__shared_state
+
+    def __str__(self):
+        return f'{self.name} is {self.age} years old'
+
+
+class Monostate:
+    _shared_state = {}
+
+    def __new__(cls, *args, **kwargs):
+        obj = super(Monostate, cls).__new__(cls, *args, **kwargs)
+        obj.__dict__ = cls._shared_state
+        return obj
+
+
+class CFO(Monostate):
+    def __init__(self):
+        self.name = ''
+        self.money_managed = 0
+
+    def __str__(self):
+        return f'{self.name} manages ${self.money_managed}bn'
+
+if __name__ == '__main__':
+    ceo1 = CEO()
+    print(ceo1)
+
+    ceo1.age = 66
+
+    ceo2 = CEO()
+    ceo2.age = 77
+    print(ceo1)
+    print(ceo2)
+
+    ceo2.name = 'Tim'
+
+    ceo3 = CEO()
+    print(ceo1, ceo2, ceo3)
+
+    cfo1 = CFO()
+    cfo1.name = 'Sheryl'
+    cfo1.money_managed = 1
+
+    print(cfo1)
+
+    cfo2 = CFO()
+    cfo2.name = 'Ruth'
+    cfo2.money_managed = 10
+    print(cfo1, cfo2, sep='\n')

src/1_patterns/2_CreationalPatterns/4_Singleton/5_singleton_testing.py

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+import unittest
+
+
+class Singleton(type):
+    _instances = {}
+
+    def __call__(cls, *args, **kwargs):
+        if cls not in cls._instances:
+            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
+        return cls._instances[cls]
+
+
+class Database(metaclass=Singleton):
+    def __init__(self):
+        self.population = {}
+        f = open('capitals.txt', 'r')
+        lines = f.readlines()
+        for i in range(0, len(lines), 2):
+            self.population[lines[i].strip()] = int(lines[i + 1].strip())
+        f.close()
+
+
+class SingletonRecordFinder:
+    def total_population(self, cities):
+        result = 0
+        for c in cities:
+            result += Database().population[c]
+        return result
+
+
+class ConfigurableRecordFinder:
+    def __init__(self, db):
+        self.db = db
+
+    def total_population(self, cities):
+        result = 0
+        for c in cities:
+            result += self.db.population[c]
+        return result
+
+
+class DummyDatabase:
+    population = {
+        'alpha': 1,
+        'beta': 2,
+        'gamma': 3
+    }
+
+    def get_population(self, name):
+        return self.population[name]
+
+class SingletonTests(unittest.TestCase):
+    def test_is_singleton(self):
+        db = Database()
+        db2 = Database()
+        self.assertEqual(db, db2)
+
+    def test_singleton_total_population(self):
+        """ This tests on a live database :( """
+        rf = SingletonRecordFinder()
+        names = ['Seoul', 'Mexico City']
+        tp = rf.total_population(names)
+        self.assertEqual(tp, 17500000 + 17400000)  # what if these change?
+
+    ddb = DummyDatabase()
+
+    def test_dependent_total_population(self):
+        crf = ConfigurableRecordFinder(self.ddb)
+        self.assertEqual(
+            crf.total_population(['alpha', 'beta']),
+            3
+        )
+
+if __name__ == '__main__':
+    unittest.main()