https://docs.python.org/dev/library/unittest.html#unittest.TestCase.assertRaises

> If only the exception and possibly the msg arguments are given, return a context manager so that the code under test can be written inline rather than as a function:
> 
>     with self.assertRaises(SomeException):
          do_something()
> 
> When used as a context manager, assertRaises() accepts the additional keyword argument msg.

Perhaps we don't need the second sentence on the `msg` argument which isn't adding anything new.

Ideally it should be more clear when the method operates in context manager mode.
("If only" and "possibly" don't play nicely together.)
Maybe along the lines of "If no callable was passed as an argument ..." ? I haven't looked in to the actual implementation yet...