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

Question

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?

Answer 1

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.

Answer 2

Do you mean something like this:

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