def checkTagAttributeSerialization(self, wrapTag):
        """
        Common implementation of L{test_serializedAttributeWithTag} and
        L{test_serializedAttributeWithDeferredTag}.

        @param wrapTag: A 1-argument callable that wraps around the attribute's
            value so other tests can customize it.
        @param wrapTag: callable taking L{Tag} and returning something
            flattenable
        """
        innerTag = tags.a('<>&"')
        outerTag = tags.img(src=wrapTag(innerTag))
        outer = self.assertFlattensImmediately(
            outerTag,
            b'<img src="&lt;a&gt;&amp;lt;&amp;gt;&amp;amp;&quot;&lt;/a&gt;" />')
        inner = self.assertFlattensImmediately(
            innerTag, b'<a>&lt;&gt;&amp;"</a>')

        # Since the above quoting is somewhat tricky, validate it by making sure
        # that the main use-case for tag-within-attribute is supported here: if
        # we serialize a tag, it is quoted *such that it can be parsed out again
        # as a tag*.
        self.assertXMLEqual(XML(outer).attrib['src'], inner)

        Common implementation of L{test_serializedAttributeWithTag} and
        L{test_serializedAttributeWithDeferredTag}.

        @param wrapTag: A 1-argument callable that wraps around the attribute's
            value so other tests can customize it.
        @param wrapTag: callable taking L{Tag} and returning something
            flattenable
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_flatten.py