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   gt            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

User · Accepted Answer

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   gt            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    gt            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  gt            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

User · Answer

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

User · Answer

Have you tried to remove  pytrace True     pytest fail exc  pytrace True    before pytest fail exc    after   Have you tried to run with  --fulltrace

User · Answer

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

User · Answer

you can try  def test exception        with pytest raises Exception  as excinfo             function that raises exception          assert str excinfo value      some info

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

There are two ways to handle exceptions in pytest   Using pytest raises to write assertions about raised exceptions 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  quot maximum recursion quot  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

User · Answer

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  quot a quot     assert False except TypeError  pass except  assert False

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

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