How do I test exceptions and errors using pytest?

Question

I have functions in my Python code that raise exceptions in response to certain conditions, and would like to confirm that they behave as expected in my pytest scripts.

Currently I have

def test_something():
    try:
        my_func(good_args)
        assert True
    except MyError as e:
        assert False
    try:
        my_func(bad_args)
        assert False
    except MyError as e:
        assert e.message == "My expected message for bad args"

but this seems cumbersome (and needs to be repeated for each case).

Is there way to test exceptions and errors using Python, or a preferred pattern for doing so?

def test_something():
    with pytest.raises(TypeError) as e:
        my_func(bad_args)
        assert e.message == "My expected message for bad args"

does not work (i.e. it passes even if I replace the assertion with assert False).

Solution

This way:

with pytest.raises(<YourException>) as exc_info:
    <your code that should raise YourException>

exception_raised = exc_info.value
<do asserts here>

Comments

[Damián Montenegro]

@raxacoricofallapatorius it works. You should refactor your code to do assertions on e out of <with .... as e: > scope.

[orome]

Can you show how I would (a) do several different tests (e.g. several calls do different functions or several calls with bad args) and (b) how to test that for no exception when there should not be one?

[Dr. Jan-Philip Gehrcke]

Pytest evolved since then. I find this more elegant, and more complete: stackoverflow.com/a/56569533/145400