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?

0

13 Answers

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.

3

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'
3

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.

Note: This comment by ilya-rusin also suggests aforementioned approach.

1

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=============================
3

you can try

def test_exception(): with pytest.raises(Exception) as excinfo: function_that_raises_exception() assert str(excinfo.value) == 'some info'
1

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
1

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)

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,

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

The top answers submitted here are useful if you are expecting an exception to be raised for your test-case. It's not very useful if your test may raise an exception and you'd want to handle it with grace in either scenario.

If you have a test-case that may (not will) raise exception, I think this might be a better option.

@python.mark.parametrize("request_query, response_code", query_response_dataset) def test_big_query_submission(request_query, response_code): try: stats = bigquery.Client().query(request_query) except Exception as e: assert False, f"Exception raised: {e}" assert stats.errors is None

This way you are covered to gracefully fail a test instead of crashing a test through a raised exception for whatever reason.

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
3

i just wrote a hook every test gets

the hook :

@pytest.hookimpl(tryfirst=True, hookwrapper=True) def pytest_runtest_makereport(item: Item, call: CallInfo):

 outcome = yield # The result after the test is completed result = outcome.get_result() if result.when == "call": if result.failed == True: else:

i have a function that updates a document with test result and adds the trace by using : result.longrepr.reprcrash.message

i Don't know if this is the best way , but it does answer the question of how to print the traceback using only pytest

@Obviously i have some other code

Have you tried to remove "pytrace=True" ?

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

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