def startElementNS(self, namespaceAndName, qname, attrs):
        """
        Gets called when we encounter a new xmlns attribute.

        @param namespaceAndName: a (namespace, name) tuple, where name
            determines which type of action to take, if the namespace matches
            L{TEMPLATE_NAMESPACE}.
        @param qname: ignored.
        @param attrs: attributes on the element being started.
        """

        filename = self.sourceFilename
        lineNumber = self.locator.getLineNumber()
        columnNumber = self.locator.getColumnNumber()

        ns, name = namespaceAndName
        if ns == TEMPLATE_NAMESPACE:
            if name == 'transparent':
                name = ''
            elif name == 'slot':
                try:
                    # Try to get the default value for the slot
                    default = attrs[(None, 'default')]
                except KeyError:
                    # If there wasn't one, then use None to indicate no
                    # default.
                    default = None
                el = slot(
                    attrs[(None, 'name')], default=default,
                    filename=filename, lineNumber=lineNumber,
                    columnNumber=columnNumber)
                self.stack.append(el)
                self.current.append(el)
                self.current = el.children
                return

        render = None

        attrs = OrderedDict(attrs)
        for k, v in items(attrs):
            attrNS, justTheName = k
            if attrNS != TEMPLATE_NAMESPACE:
                continue
            if justTheName == 'render':
                render = v
                del attrs[k]

        # nonTemplateAttrs is a dictionary mapping attributes that are *not* in
        # TEMPLATE_NAMESPACE to their values.  Those in TEMPLATE_NAMESPACE were
        # just removed from 'attrs' in the loop immediately above.  The key in
        # nonTemplateAttrs is either simply the attribute name (if it was not
        # specified as having a namespace in the template) or prefix:name,
        # preserving the xml namespace prefix given in the document.

        nonTemplateAttrs = OrderedDict()
        for (attrNs, attrName), v in items(attrs):
            nsPrefix = self.prefixMap.get(attrNs)
            if nsPrefix is None:
                attrKey = attrName
            else:
                attrKey = '%s:%s' % (nsPrefix, attrName)
            nonTemplateAttrs[attrKey] = v

        if ns == TEMPLATE_NAMESPACE and name == 'attr':
            if not self.stack:
                # TODO: define a better exception for this?
                raise AssertionError(
                    '<{%s}attr> as top-level element' % (TEMPLATE_NAMESPACE,))
            if 'name' not in nonTemplateAttrs:
                # TODO: same here
                raise AssertionError(
                    '<{%s}attr> requires a name attribute' % (TEMPLATE_NAMESPACE,))
            el = Tag('', render=render, filename=filename,
                     lineNumber=lineNumber, columnNumber=columnNumber)
            self.stack[-1].attributes[nonTemplateAttrs['name']] = el
            self.stack.append(el)
            self.current = el.children
            return

        # Apply any xmlns attributes
        if self.xmlnsAttrs:
            nonTemplateAttrs.update(OrderedDict(self.xmlnsAttrs))
            self.xmlnsAttrs = []

        # Add the prefix that was used in the parsed template for non-template
        # namespaces (which will not be consumed anyway).
        if ns != TEMPLATE_NAMESPACE and ns is not None:
            prefix = self.prefixMap[ns]
            if prefix is not None:
                name = '%s:%s' % (self.prefixMap[ns],name)
        el = Tag(
            name, attributes=OrderedDict(nonTemplateAttrs), render=render,
            filename=filename, lineNumber=lineNumber,
            columnNumber=columnNumber)
        self.stack.append(el)
        self.current.append(el)
        self.current = el.children

        Gets called when we encounter a new xmlns attribute.

        @param namespaceAndName: a (namespace, name) tuple, where name
            determines which type of action to take, if the namespace matches
            L{TEMPLATE_NAMESPACE}.
        @param qname: ignored.
        @param attrs: attributes on the element being started.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/template.py