Repository files navigation

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

fromthreadingimportThreaddeftest_assert():defrun():assertFalseThread(target=run).start()

This test will pass as theAssertionError 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

fromthreadingimportThreaddeftest_assert(reraise):defrun():withreraise:assertFalseThread(target=run).start()

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

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

defrun():withreraise:assertFalseThread(target=run).start()

can also be written as

defrun():assertFalseThread(target=reraise.wrap(run)).start()

or even

@reraise.wrapdefrun():assertFalseThread(target=run).start()

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

deftest_assert(reraise):defrun():withreraise:assertFalsereraise()# This will not raise anything yett=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 awith reraise block since the last call, it will be raised on the next call.

When thereraise 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.

deftest_assert(reraise):defrun():withreraise:assertFalsefor_inrange(2):Thread(target=run).start()

By default, thereraise 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, usereraise(catch=True) instead ofreraise:

deftest_assert(reraise):defrun():withreraise(catch=True):assertFalseprint("I'm alive!")Thread(target=run).start()

Note that you cannot usereraise() (without thecatch argument) as a context manager, as it is used to raise exceptions.

Ifreraise captures an exception and the main thread raises an exception as well, the exception captured byreraise 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 reportassert False, notassert "foo" == "bar".

deftest_assert(reraise):defrun():withreraise:assertFalse# This will be reportedt=Thread(target=run)t.start()t.join()assert"foo"=="bar"# This won't

reraise provides anexception 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 ofreraise.exception and resets it toNone so that the exception will not be raised anymore.

Here's a quick demonstration test case that passes:

deftest_assert(reraise):defrun():withreraise:assertFalset=Thread(target=run)t.start()t.join()# Return the captured exception:asserttype(reraise.exception)isAssertionError# This won't do anything, since an exception has already been captured:reraise.exception=Exception()# Return the exception and set `reraise.exception` to None:asserttype(reraise.reset())isAssertionError# `Reraise` will not fail the test case becauseassertreraise.exceptionisNone