def _formatRoot(self, obj):
        """
        Convert an object from C{self._roots} to a string suitable for
        inclusion in a render-traceback (like a normal Python traceback, but
        can include "frame" source locations which are not in Python source
        files).

        @param obj: Any object which can be a render step I{root}.
            Typically, L{Tag}s, strings, and other simple Python types.

        @return: A string representation of C{obj}.
        @rtype: L{str}
        """
        # There's a circular dependency between this class and 'Tag', although
        # only for an isinstance() check.
        from twisted.web.template import Tag

        if isinstance(obj, (bytes, str, unicode)):
            # It's somewhat unlikely that there will ever be a str in the roots
            # list.  However, something like a MemoryError during a str.replace
            # call (eg, replacing " with &quot;) could possibly cause this.
            # Likewise, UTF-8 encoding a unicode string to a byte string might
            # fail like this.
            if len(obj) > 40:
                if isinstance(obj, unicode):
                    ellipsis = u'<...>'
                else:
                    ellipsis = b'<...>'
                return ascii(obj[:20] + ellipsis + obj[-20:])
            else:
                return ascii(obj)
        elif isinstance(obj, Tag):
            if obj.filename is None:
                return 'Tag <' + obj.tagName + '>'
            else:
                return "File \"%s\", line %d, column %d, in \"%s\"" % (
                    obj.filename, obj.lineNumber,
                    obj.columnNumber, obj.tagName)
        else:
            return ascii(obj)

        Convert an object from C{self._roots} to a string suitable for
        inclusion in a render-traceback (like a normal Python traceback, but
        can include "frame" source locations which are not in Python source
        files).

        @param obj: Any object which can be a render step I{root}.
            Typically, L{Tag}s, strings, and other simple Python types.

        @return: A string representation of C{obj}.
        @rtype: L{str}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/error.py