pytest-reraise

Let's assume you are writing a pytest test case that includes assertions in another thread, roughly like this:

from threading import Thread

def test_assert():

    def run():
        assert False

    Thread(target=run).start()

This test will pass as the AssertionError is not raised in the main thread. pytest-reraise is here to help you capture the exception and raise it in the main thread:

pip install pytest-reraise

from threading import Thread

def test_assert(reraise):

    def run():
        with reraise:
            assert False

    Thread(target=run).start()

The above test will fail as pytest-reraise captures the exception and raises it at the end of the test case.

Advanced Usage and Special Cases

Wrapping Functions

Instead of using the reraise context manager in a function, you can also wrap the entire function with it via the reraise.wrap() method. Hence, the example

def run():
    with reraise:
        assert False

Thread(target=run).start()

can also be written as

def run():
    assert False

Thread(target=reraise.wrap(run)).start()

or even

@reraise.wrap
def run():
    assert False

Thread(target=run).start()

Manual Re-raising

By default, the captured exception (if any) is raised at the end of the test case. If you want to raise it before then, call reraise() in your test case. If an exception has been raised within a with reraise block by then, reraise() will raise it right away:

def test_assert(reraise):

    def run():
        with reraise:
            assert False

    reraise() # This will not raise anything yet

    t = Thread(target=run)
    t.start()
    t.join()

    reraise() # This will raise the assertion error

As seen in the example above, reraise() can be called multiple times during a test case. Whenever an exception has been raised in a with reraise block since the last call, it will be raised on the next call.

Multiple Exceptions

When the reraise context manager is used multiple times in a single test case, only the first-raised exception will be re-raised in the end. In the below example, both threads raise an exception but only one of these exceptions will be re-raised.

def test_assert(reraise):

    def run():
        with reraise:
            assert False

    for _ in range(2):
        Thread(target=run).start()

Catching Exceptions

By default, the reraise context manager does not catch exceptions, so they will not be hidden from the thread in which they are raised. If you want to change this, use reraise(catch=True) instead of reraise:

def test_assert(reraise):

    def run():
        with reraise(catch=True):
            assert False
        print("I'm alive!")

    Thread(target=run).start()

Note that you cannot use reraise() (without the catch argument) as a context manager, as it is used to raise exceptions.

Exception Priority

If reraise captures an exception and the main thread raises an exception as well, the exception captured by reraise will mask the main thread's exception unless that exception was already re-raised. The objective behind this is that the outcome of the main thread often depends on the work performed in other threads. Thus, failures in in other threads are likely to cause failures in the main thread, and other threads' exceptions (if any) are of greater importance for the developer than main thread exceptions.

The example below will report assert False, not assert "foo" == "bar".

def test_assert(reraise):

    def run():
        with reraise:
            assert False # This will be reported

    t = Thread(target=run)
    t.start()
    t.join()

    assert "foo" == "bar" # This won't

Accessing and Modifying Exceptions

reraise provides an exception property to retrieve the exception that was captured, if any. reraise.exception can also be used to assign an exception if no exception has been captured yet. In addition to that, reraise.reset() returns the value of reraise.exception and resets it to None so that the exception will not be raised anymore.

Here's a quick demonstration test case that passes:

def test_assert(reraise):

    def run():
        with reraise:
            assert False

    t = Thread(target=run)
    t.start()
    t.join()

    # Return the captured exception:
    assert type(reraise.exception) is AssertionError

    # This won't do anything, since an exception has already been captured:
    reraise.exception = Exception()

    # Return the exception and set `reraise.exception` to None:
    assert type(reraise.reset()) is AssertionError

    # `Reraise` will not fail the test case because
    assert reraise.exception is None