How can I test 'warnings' from testsuite?

[billiejoex]

Hi there,
into a module of mine I 'warn' a message if a certain situation
occurs:


def add_anonymous_user(permissions=('r'):
      if 'w' in p:
          import warnings
          warnings.warn("it's not rencommended assigning 'w'
permission to anonymous user.", RuntimeWarning, stacklevel=2)


I'd like to test such event from test suite ("fail test if warn is not
raised") but don't know how.

Any suggestion?