In [55]: actual, required = {'a', 'm'}, {'a', 'm', 'u'}
In [56]: actual - required
Out[56]: set()

In [57]: actual, required = {'a', 'm', 'u'}, {'a', 'm', 'u'}
In [58]: actual - required
Out[58]: set()

In [59]: actual, required = {'a', 'm', 'u', 'd'}, {'a', 'm', 'u'}
In [60]: actual - required
Out[60]: {'d'}

In [72]: actual, required = {'a', 'm'}, {'a', 'm', 'u'}
In [73]: actual.intersection(required) == required
Out[73]: False

In [74]: actual, required = {'a', 'm', 'u'}, {'a', 'm', 'u'}
In [75]: actual.intersection(required) == required
Out[75]: True

In [76]: actual, required = {'a', 'm', 'u', 'd'}, {'a', 'm', 'u'}
In [77]: actual.intersection(required) == required
Out[77]: True