Question

I have a unit test that checks for exception in constructor:

import unittest
from jaboci import Jacobi

class TestJacobi(unittest.TestCase):

    def test_even(self):
        a = 11
        n = 12
        Jacobi(a, n)
        self.assertRaises(ValueError, Jacobi, a, n)

if __name__ == '__main__':
    unittest.main()

The class under test:

class Jacobi:

    def __init__(self, a, n):
        self.a = a
        self.n = n
        if n % 2 == 0:
            raise ValueError("N must be odd.")

When i run unittest with -m unittest discover, the test fails:

E
======================================================================
ERROR: test_even (test_jacobi.TestJacobi)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/prasopes/prg/python/PycharmProjects/jacobi_symbol/test_jacobi.py", line 9, in test_even
    Jacobi(a, n)
  File "/home/prasopes/prg/python/PycharmProjects/jacobi_symbol/jaboci.py", line 7, in __init__
    raise ValueError("N must be odd.")
ValueError: N must be odd.

----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (errors=1)

Answer 1

You call Jacobi(a, n) before self.assertRaises(ValueError, Jacobi, a, n). The exception you get is from this first call, so the test immediately fails. It never reaches the line with assertRaises.