[python] 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?

you can try

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

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

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

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.

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

• Using pytest.raises function

• Using pytest.mark.xfail decorator

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.

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=============================

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

Do you mean something like this:

def test_raises():
    with pytest.raises(Exception) as execinfo:   
        raise Exception('some info')
    # these asserts are identical; you can use either one   
    assert execinfo.value.args[0] == 'some info'
    assert str(execinfo.value) == 'some info'

If you want to test for a specific error type, use a combination of try, catch and raise:

#-- test for TypeError
try:
  myList.append_number("a")
  assert False
except TypeError: pass
except: assert False

Have you tried to remove "pytrace=True" ?

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

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

There are two ways to handle exceptions in pytest:

1. Using pytest.raises to write assertions about raised exceptions
2. Using @pytest.mark.xfail

1. Using pytest.raises

From the docs:

In order to write assertions about raised exceptions, you can use pytest.raises as a context manager

Examples:

Asserting just an exception:

import pytest


def test_zero_division():
    with pytest.raises(ZeroDivisionError):
        1 / 0

with pytest.raises(ZeroDivisionError) says that whatever is in the next block of code should raise a ZeroDivisionError exception. If no exception is raised, the test fails. If the test raises a different exception, it fails.

If you need to have access to the actual exception info:

import pytest

def f():
    f()

def test_recursion_depth():
    with pytest.raises(RuntimeError) as excinfo:
        f()
    assert "maximum recursion" in str(excinfo.value)

excinfo is a ExceptionInfo instance, which is a wrapper around the actual exception raised. The main attributes of interest are .type, .value and .traceback.

2. Using @pytest.mark.xfail

It is also possible to specify a raises argument to pytest.mark.xfail.

import pytest

@pytest.mark.xfail(raises=IndexError)
def test_f():
    l = [1, 2, 3]
    l[10]

@pytest.mark.xfail(raises=IndexError) says that whatever is in the next block of code should raise an IndexError exception. If an IndexError is raised, test is marked as xfailed (x). If no exception is raised, the test is marked as xpassed (X). If the test raises a different exception, it fails.

Notes:

• 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 or bugs in dependencies.

• You can pass a match keyword parameter to the context-manager (pytest.raises) to test that a regular expression matches on the string representation of an exception. (see more)