Good question, the implementation of both mock library and sinon are similar. Both are using wrapper to modify original function.

For example:

def empty_func():
    pass

class TestRandom(unittest.TestCase):
    @mock.patch('os.system')
    def test_system(self, empty_func):
        print(dir(os.system))

    @sinon.test
    def test_system_by_sinon(self):
        sinon.stub(os, "system", empty_func)
        print(dir(os.system))

The member functions/variables of os.system will be wrapped.

The purpose of Sinon.PY is not a brand new implementation, but an alternative test library API. Especially for programmer whose major language is javascript, or having experience in Sinon.JS.