Assert exception message?

Question

I'm using pytest in a project with a good many custom exceptions.

pytest provides a handy syntax for checking that an exception has been raised, however, I'm not aware of one that asserts that the correct exception message has been raised.

Say I had a CustomException that prints "boo!", how could I assert that "boo!" has indeed been printed and not, say, "<unprintable CustomException object>"?

#errors.py
class CustomException(Exception):
    def __str__(self): return "ouch!"

#test.py
import pytest, myModule

def test_custom_error(): # SHOULD FAIL
    with pytest.raises(myModule.CustomException):
        raise myModule.CustomException == "boo!"

`a CustomException that prints "boo!"` - do you mean a custom Exception whose string representation is "boo"? e.g. you want to `assert str(exc) == "Boo!"`? — Tom Dalton, Aug 19 '19 at 16:21
If you actually want to capture stdout or stderr, then you might want to look at the capsys fixture: https://docs.pytest.org/en/latest/capture.html#accessing-captured-output-from-a-test-function — Tom Dalton, Aug 19 '19 at 16:22
Take a look at this: https://stackoverflow.com/a/23514853/2372812, `excinfo.value` is the actual exeption raised so you can do `assert str(excinfo.value) == "Boo"` — Tom Dalton, Aug 19 '19 at 16:24

Tom Dalton · Accepted Answer · 2021-05-30T10:01:56.967

23

I think what you're looking for is:

def failer():
    raise myModule.CustomException()

def test_failer():
    with pytest.raises(myModule.CustomException) as excinfo:
        failer()

    assert str(excinfo.value) == "boo!"

edited May 30 '21 at 10:01

answered Aug 19 '19 at 16:25

Tom Dalton

6,122
24
35

That is indeed what I'm looking for, thank you very much! – snazzybouche Aug 20 '19 at 08:24
4

Note: you have to access it outside the `with` block as in this answer. You can't access it after `failer()` – J.Kirk. Mar 19 '21 at 17:55
That's true, and it's because any code directly after `failer()` will never be executed - the exception means it wont' get that far. – Tom Dalton Feb 07 '22 at 15:11

score 8 · Answer 2 · answered Jul 14 '22 at 08:47

8

You can use match keyword in raises. Try something like

with pytest.raises(
        RuntimeError, match=<error string here>
    ):
     pass

answered Jul 14 '22 at 08:47

manu datta

141
2
4

Assert exception message?

2 Answers2