assertRaises not working with Futures

Question

I changed a call to bar which returned synchronously to returning a Future. When the method of bar actually gets executed it raises an ValueError. The unit tests actually ensured that.

Now changing

self.assertRaises(ValueError, foo.bar("/my/invalid/path"))

to

self.assertRaises(ValueError, fut.result())

no longer is a valid assert for some reason.

The whole new code is:

fut = foo.bar("/my/invalid/path")
self.assertIsNotNone(fut)
self.assertRaises(ValueError, fut.result())

The stack trace then is:

"test.py", line 25, in test_load_invalid_res
self.assertRaises(ValueError, fut.result())
File "/usr/lib/python3.5/concurrent/futures/_base.py", line 398, in result
  return self.__get_result()
File "/usr/lib/python3.5/concurrent/futures/_base.py", line 357, in __get_result
  raise self._exception
File "/usr/lib/python3.5/concurrent/futures/thread.py", line 55, in run
  result = self.fn(*self.args, **self.kwargs)
ValueError: Could not find resource in resource path: /my/invalid/path

But the stack trace is actually telling me that there was a ValueError!? Why is it propagading beyond assertRaises?

EDIT:

The following code does not work:

try:
  fut.result()
except ValueError as e:
  pass

What works is e.g.:

try:
  fut.result()
except Exception as e:
  self.assertIsInstance(e, ValueError)

Answer 1

You are calling the method and passing the result to assertRaises. Since the error happens before assertRaises is invoked, it will not catch the exception.

assertRaises takes a callable which it calls itself within a try/except. Your code should be:

self.assertRaises(ValueError, fut.result)

without the calling parens. Alternatively you can use it as a context manager:

with self.assertRaises(ValueError):
    fut.result()