How to properly assert that an exception gets raised in pytest?
Solution 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.
Solution 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'
Solution 3
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=============================
Solution 4
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.
Solution 5
you can try
def test_exception():
with pytest.raises(Exception) as excinfo:
function_that_raises_exception()
assert str(excinfo.value) == 'some info'
Gill Bates
Updated on July 22, 2022Comments
-
Gill Bates almost 2 years
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?