Pytest No exception is raised

---

examples/pytest/fib6/fibonacci.py

def fib(n):
    if n < 1:
        return None
    a, b = 1, 1
    for _ in range(1, n):
        a, b = b, a+b
    return a

examples/pytest/fib6/test_fibonacci.py

import pytest
from fibonacci import fib

def test_fib():
    assert fib(10) == 55

def test_fib_negative():
    with pytest.raises(Exception) as err:
        fib(-1)
    assert err.type == ValueError
    assert str(err.value) == 'Invalid parameter -1'

def test_fib_negative_again():
    with pytest.raises(ValueError) as err:
        fib(-1)
    assert str(err.value) == 'Invalid parameter -1'

============================= test session starts ==============================
platform linux -- Python 3.8.6, pytest-6.1.2, py-1.9.0, pluggy-0.13.1
rootdir: /home/gabor/work/slides/python/examples/pytest/fib6
plugins: flake8-1.0.6, dash-1.17.0
collected 3 items

test_fibonacci.py .FF                                                    [100%]

=================================== FAILURES ===================================
______________________________ test_fib_negative _______________________________

    def test_fib_negative():
        with pytest.raises(Exception) as err:
>           fib(-1)
E           Failed: DID NOT RAISE <class 'Exception'>

test_fibonacci.py:9: Failed
___________________________ test_fib_negative_again ____________________________

    def test_fib_negative_again():
        with pytest.raises(ValueError) as err:
>           fib(-1)
E           Failed: DID NOT RAISE <class 'ValueError'>

test_fibonacci.py:15: Failed
=========================== short test summary info ============================
FAILED test_fibonacci.py::test_fib_negative - Failed: DID NOT RAISE <class 'E...
FAILED test_fibonacci.py::test_fib_negative_again - Failed: DID NOT RAISE <cl...
========================= 2 failed, 1 passed in 0.03s ==========================