How to properly assert that an exception gets raised in pytest?

Code:

# coding=utf-8
import pytest


def whatever():
    return 9/0

def test_whatever():
    try:
        whatever()
    except ZeroDivisionError as exc:
        pytest.fail(exc, pytrace=True)

Output:

================================ test session starts =================================
platform linux2 -- Python 2.7.3 -- py-1.4.20 -- pytest-2.5.2
plugins: django, cov
collected 1 items 

pytest_test.py F

====================================== FAILURES ======================================
___________________________________ test_whatever ____________________________________

    def test_whatever():
        try:
            whatever()
        except ZeroDivisionError as exc:
>           pytest.fail(exc, pytrace=True)
E           Failed: integer division or modulo by zero

pytest_test.py:12: Failed
============================== 1 failed in 1.16 seconds ==============================

How to make pytest print traceback, so I would see where in the whatever function an exception was raised?

Murilo Giacometti

pytest.raises(Exception) is what you need.

Code

import pytest

def test_passes():
    with pytest.raises(Exception) as e_info:
        x = 1 / 0

def test_passes_without_info():
    with pytest.raises(Exception):
        x = 1 / 0

def test_fails():
    with pytest.raises(Exception) as e_info:
        x = 1 / 1

def test_fails_without_info():
    with pytest.raises(Exception):
        x = 1 / 1

# Don't do this. Assertions are caught as exceptions.
def test_passes_but_should_not():
    try:
        x = 1 / 1
        assert False
    except Exception:
        assert True

# Even if the appropriate exception is caught, it is bad style,
# because the test result is less informative
# than it would be with pytest.raises(e)
# (it just says pass or fail.)

def test_passes_but_bad_style():
    try:
        x = 1 / 0
        assert False
    except ZeroDivisionError:
        assert True

def test_fails_but_bad_style():
    try:
        x = 1 / 1
        assert False
    except ZeroDivisionError:
        assert True

Output

============================================================================================= test session starts ==============================================================================================
platform linux2 -- Python 2.7.6 -- py-1.4.26 -- pytest-2.6.4
collected 7 items 

test.py ..FF..F

=================================================================================================== FAILURES ===================================================================================================
__________________________________________________________________________________________________ test_fails __________________________________________________________________________________________________

    def test_fails():
        with pytest.raises(Exception) as e_info:
>           x = 1 / 1
E           Failed: DID NOT RAISE

test.py:13: Failed
___________________________________________________________________________________________ test_fails_without_info ____________________________________________________________________________________________

    def test_fails_without_info():
        with pytest.raises(Exception):
>           x = 1 / 1
E           Failed: DID NOT RAISE

test.py:17: Failed
___________________________________________________________________________________________ test_fails_but_bad_style ___________________________________________________________________________________________

    def test_fails_but_bad_style():
        try:
            x = 1 / 1
>           assert False
E           assert False

test.py:43: AssertionError
====================================================================================== 3 failed, 4 passed in 0.02 seconds ======================================================================================

Note that e_info saves the exception object so you can extract details from it. For example, if you want to check the exception call stack or another nested exception inside.

simpleranchero

Do you mean something like this:

def test_raises():
    with pytest.raises(Exception) as excinfo:   
        raise Exception('some info')   
    assert excinfo.value.message == 'some info'

veri_pudcha_coder

There are two ways to handle these kind of cases in pytest:

Using pytest.raises function
Using pytest.mark.xfail decorator

Usage of pytest.raises:

def whatever():
    return 9/0
def test_whatever():
    with pytest.raises(ZeroDivisionError):
        whatever()

Usage of pytest.mark.xfail:

@pytest.mark.xfail(raises=ZeroDivisionError)
def test_whatever():
    whatever()

Output of pytest.raises:

============================= test session starts ============================
platform linux2 -- Python 2.7.10, pytest-3.2.3, py-1.4.34, pluggy-0.4.0 -- 
/usr/local/python_2.7_10/bin/python
cachedir: .cache
rootdir: /home/user, inifile:
collected 1 item

test_fun.py::test_whatever PASSED


======================== 1 passed in 0.01 seconds =============================

Output of pytest.xfail marker:

============================= test session starts ============================
platform linux2 -- Python 2.7.10, pytest-3.2.3, py-1.4.34, pluggy-0.4.0 -- 
/usr/local/python_2.7_10/bin/python
cachedir: .cache
rootdir: /home/user, inifile:
collected 1 item

test_fun.py::test_whatever xfail

======================== 1 xfailed in 0.03 seconds=============================

As the documentation says:

Using pytest.raises is likely to be better for cases where you are testing exceptions your own code is deliberately raising, whereas using @pytest.mark.xfail with a check function is probably better for something like documenting unfixed bugs (where the test describes what “should” happen) or bugs in dependencies.

you can try

def test_exception():
    with pytest.raises(Exception) as excinfo:   
        function_that_raises_exception()   
    assert str(excinfo.value) == 'some info'

Right way is using pytest.raises but I found interesting alternative way in comments here and want to save it for future readers of this question:

try:
    thing_that_rasises_typeerror()
    assert False
except TypeError:
    assert True

This solution is what we are using:

def test_date_invalidformat():
    """
    Test if input incorrect data will raises ValueError exception
    """
    date = "06/21/2018 00:00:00"
    with pytest.raises(ValueError):
        app.func(date) #my function to be tested

Please refer to pytest, https://docs.pytest.org/en/latest/reference.html#pytest-raises

Better practice will be using a class that inherit unittest.TestCase and running self.assertRaises.

For example:

import unittest


def whatever():
    return 9/0


class TestWhatEver(unittest.TestCase):

    def test_whatever():
        with self.assertRaises(ZeroDivisionError):
            whatever()

Then you would execute it by running:

pytest -vs test_path

pytest constantly evolves and with one of the nice changes in the recent past it is now possible to simultaneously test for

the exception type (strict test)
the error message (strict or loose check using a regular expression)

Two examples from the documentation:

with pytest.raises(ValueError, match='must be 0 or None'):
    raise ValueError('value must be 0 or None')

with pytest.raises(ValueError, match=r'must be \d+$'):
    raise ValueError('value must be 42')

I have been using that approach in a number of projects and like it very much.

Have you tried to remove "pytrace=True" ?

pytest.fail(exc, pytrace=True) # before
pytest.fail(exc) # after

Have you tried to run with '--fulltrace' ?

来源：https://stackoverflow.com/questions/23337471/how-to-properly-assert-that-an-exception-gets-raised-in-pytest

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