[Python] How to fix the fibonacci code without using recursion? [closed]

I'm trying to understand how to write the Fibonacci sequence.

def fibonacci(n):
if not isinstance(n, int):
return None
if n < 0:
return None

if n == 0:#F_0
return 0
elif n == 1 or n == 2: #F_1 and F_2
return 1

a, b = 1, 1 #F_3
for i in range(3,n + 1):
a, b = a + b, a
return a


Basically, if I calculate it by hand, I get the correct values, but the unit test gives an error.

#Test

import unittest

class Test(unittest.TestCase):

def test(self):
self.assertEqual(fibonacci(1), 1, "Program is calculating the wrong value!")
self.assertEqual(fibonacci(2), 1, "Program is calculating the wrong value!")
self.assertEqual(fibonacci(3), 2, "Program is calculating the wrong value!")
self.assertEqual(fibonacci(4), 3, "Program is calculating the wrong value!")
self.assertEqual(fibonacci(5), 5, "Program is calculating the wrong value!")

def test_errors(self):
self.assertEqual(fibonacci(-1), None, "Program is calculating the wrong value!")
self.assertEqual(fibonacci(-1.0), None, "Program is calculating the wrong value!")
self.assertEqual(fibonacci(1.0), None, "Program is calculating the wrong value!")
self.assertEqual(fibonacci("Hallo"), None, "Program is calculating the wrong value!")


if __name__ == '__main__':
unittest.main(argv=['first-arg-is-ignored'], exit=False)


For example, a, b = 1, 1, which I think should be correct, gives the error:

self.assertEqual(fibonacci(4), 3, "Program is calculating the wrong value!")
~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 2 != 3 : Program is calculating the wrong value!

and a, b = 1, 0 self.assertEqual(fibonacci(3), 2, "Program is calculating the wrong value!")
~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 1 != 2 : Program is calculating the wrong value!

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