def test_commit(self):

       self.wc.checkout(self.repo)

       p = self.wc.join("a_file")

->     p.write("test file")

       p.add()

       rev = self.wc.commit("some test")

       assert p.info().rev == 1

       assert rev == 1

       rev = self.wc.commit()

       assert rev is None