def deprecated_call(func, *args, **kwargs):

    """ assert that calling func(*args, **kwargs)

        triggers a DeprecationWarning. 

    """ 

    l = []

    oldwarn = py.std.warnings.warn_explicit

    def warn_explicit(*args, **kwargs): 

        l.append(args) 

        oldwarn(*args, **kwargs)

    py.magic.patch(py.std.warnings, 'warn_explicit', warn_explicit)

    try:

        _ = func(*args, **kwargs)

    finally:

        py.magic.revert(py.std.warnings, 'warn_explicit')

    assert l