def test_partialWrite(self):
        """
        The literal returns L{None} when given less data than the
        literal requires.
        """
        literal = self.literalFactory(1024, self.deferred)
        self.assertIs(None, literal.write(b"incomplete"))
        self.assertNoResult(self.deferred)

        The literal returns L{None} when given less data than the
        literal requires.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_imap.py