Return nth fibonacci number using memoization, Python 3

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"""
Return nth fibonacci number using memoization

Author: Jayesh Chandrapal
Version: 1.0

"""

import unittest

class Fibonacci:
    def __init__(self):
        self.fib_numbers = {0: 0, 1: 1}
        
    def fib(self, n):
        if n in self.fib_numbers.keys():
            return self.fib_numbers[n]
        else:
            self.fib_numbers[n] = self.__fib(n)
            
        return self.fib_numbers[n]
    
    def __fib(self, n):
        if n == 0:
            return 0
        if n == 1:
            return 1
        
        if n not in self.fib_numbers:
            self.fib_numbers[n] = self.__fib(n - 1) + self.__fib(n - 2)
            
        return self.fib_numbers[n]

"""
Test cases
"""
class TestCases(unittest.TestCase):
    def setUp(self):
        self.app = Fibonacci()
        
    def tearDown(self):
        self.app = None
        
    def test0(self):
        self.assertEqual(8, self.app.fib(6))
        
    def test1(self):
        self.assertEqual(5, self.app.fib(5))
        
    def test2(self):
        self.assertEqual(3, self.app.fib(4))
        
    def test3(self):
        self.assertEqual(2, self.app.fib(3))
        
    def test4(self):
        self.assertEqual(1, self.app.fib(2))
        
    def test5(self):
        self.assertEqual(1, self.app.fib(1))
        
    def test6(self):
        self.assertEqual(0, self.app.fib(0))
        
        
if __name__ == "__main__":
    unittest.main()

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Absolute running time: 0.14 sec, cpu time: 0.04 sec, memory peak: 8 Mb, absolute service time: 0,14 sec