def test_copy(self):
        """
        L{Headers.copy} creates a new independent copy of an existing
        L{Headers} instance, allowing future modifications without impacts
        between the copies.
        """
        h = Headers()
        h.setRawHeaders(b'test', [b'foo'])
        i = h.copy()
        self.assertEqual(i.getRawHeaders(b'test'), [b'foo'])
        h.addRawHeader(b'test', b'bar')
        self.assertEqual(i.getRawHeaders(b'test'), [b'foo'])
        i.addRawHeader(b'test', b'baz')
        self.assertEqual(h.getRawHeaders(b'test'), [b'foo', b'bar'])

        L{Headers.copy} creates a new independent copy of an existing
        L{Headers} instance, allowing future modifications without impacts
        between the copies.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_http_headers.py