Fail unless a warning of class warnClass is triggered
by the callable when invoked with specified positional and
keyword arguments. If a different type of warning is
triggered, it will not be handled: depending on the other
warning filtering rules in effect, it might be silenced, printed
out, or raised as an exception.
If called with the callable and arguments omitted, will return a
context object used like this::
with self.assertWarns(SomeWarning):
do_something()
An optional keyword argument 'msg' can be provided when assertWarns
is used as a context object.
The context manager keeps a reference to the first matching
warning as the 'warning' attribute; similarly, the 'filename'
and 'lineno' attributes give you information about the line
of Python code from which the warning was triggered.
This allows you to inspect the warning after the assertion::
with self.assertWarns(SomeWarning) as cm:
do_something()
the_warning = cm.warning
self.assertEqual(the_warning.some_attribute, 147)