Pytest: Why did not raise with ZeroDivisionError??

[re4lfl0w]

exception.py

def divide_anti_pattern(a, b):
    try:
        result = a / b
    except:
        result = None
    return result


def divide_best_practice(a, b):
    result = None
    try:
        result = a / b
    except ZeroDivisionError:
        print('Type error: division by 0.')
    except TypeError:
        # E.g., if b is a string
        print("Type error: division by '{0}'.".format(b))
    except Exception as e:
        # handle any other exception
        print("Error '{0}' occurred. Arguments {1}.".format(e.args))
    else:
        # Excute if no exception occurred
        print('No errors')
    finally:
        # Excute always
        if result is None:
            result = 0
    return result

test_exception.py

import pytest
from pytest import raises

from tests.anti_patterns.exception import divide_anti_pattern, divide_best_practice


class TestDivideAntiPattern:
    def test_divide_anti_pattern(self):
        assert divide_anti_pattern(5, 5) == 1

    def test_divide_anti_pattern_with_divide_zero(self):
        assert divide_anti_pattern(5, 0) == None


class TestDivideBestPractice:
    def test_divide_best_practice(self):
        assert divide_best_practice(5, 4) == 1.25

    def test_divide_best_practice_with_divide_zero(self):
        with raises(ZeroDivisionError):
            divide_best_practice(15, 0)

    def test_divide_best_practice_with_divide_zero2(self):
        with raises(ZeroDivisionError):
            print(1/0)

(reservation) untitled1 (master) ✗ py.test tests/anti_patterns/test_exception.py
================================================= test session starts =================================================
platform darwin -- Python 3.4.3, pytest-2.8.5, py-1.4.31, pluggy-0.3.1
rootdir: /Users/re4lfl0w/PycharmProjects/untitled1/tests, inifile: pytest.ini
plugins: cov-2.2.0, django-2.9.1, xdist-1.13.1
collected 5 items 

tests/anti_patterns/test_exception.py ...F.

====================================================== FAILURES =======================================================
__________________________ TestDivideBestPractice.test_divide_best_practice_with_divide_zero __________________________

self = <test_exception.TestDivideBestPractice object at 0x1065bada0>

    def test_divide_best_practice_with_divide_zero(self):
        with raises(ZeroDivisionError):
>           divide_best_practice(15, 0)
E           Failed: DID NOT RAISE

tests/anti_patterns/test_exception.py:21: Failed
------------------------------------------------ Captured stdout call -------------------------------------------------
Type error: division by 0.

[RonnyPfannschmidt]

If you catch it, raises no longer Sees it,

[nicoddemus]

If you just call divide_best_practice outside a test, you will see that it won't raise ZeroDivisionError either. That's because, as @RonnyPfannschmidt said, you are catching it and just printing a message, effectively swallowing it to the caller. So pytest is correct here.

If you want the caller to see the exception, you have to re-raise it yourself:

    except ZeroDivisionError:
        print('Type error: division by 0.')
        raise

[re4lfl0w]

@RonnyPfannschmidt Thank you.
@nicoddemus Thank you. I realized your example code. I forget 'raise'. ok. understand. Always write 'raise' in except??

[nicoddemus]

If you want to propagate the exception to the caller, then yes. The are situations where you might want to handle the exception locally though.