Что за код

def start_response(self, status, headers,exc_info=None):
        """'start_response()' callable as specified by PEP 3333"""
        if exc_info:
            try:
                if self.headers_sent:
                    # Re-raise original exception if headers sent
                    raise exc_info[0](exc_info[1]).with_traceback(exc_info[2])
            finally:
                exc_info = None        # avoid dangling circular ref
        elif self.headers is not None:
            raise AssertionError("Headers already set!")
 
        self.status = status
        self.headers = self.headers_class(headers)
        status = self._convert_string_type(status, "Status")
        assert len(status)>=4,"Status must be at least 4 characters"
        assert status[:3].isdigit(), "Status message must begin w/3-digit code"
        assert status[3]==" ", "Status message must have a space after code"
 
        if __debug__:
            for name, val in headers:
                name = self._convert_string_type(name, "Header name")
                val = self._convert_string_type(val, "Header value")
                assert not is_hop_by_hop(name),\
                       f"Hop-by-hop header, '{name}: {val}', not allowed"
        return self.write

def write(self, data):
        """'write()' callable as specified by PEP 3333"""
 
        assert type(data) is bytes, \
            "write() argument must be a bytes instance"
 
        if not self.status:
            raise AssertionError("write() before start_response()")
 
        elif not self.headers_sent:
            # Before the first output, send the stored headers
            self.bytes_sent = len(data)    # make sure we know content-length
            self.send_headers()
        else:
            self.bytes_sent += len(data)
 
        # XXX check Content-Length and truncate if too many bytes written?
        self._write(data)
        self._flush()

class WSGIHandler(base.BaseHandler):
    request_class = WSGIRequest
 
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.load_middleware()
    
 
    def __call__(self, environ, start_response):
        set_script_prefix(get_script_name(environ))
        signals.request_started.send(sender=self.__class__, environ=environ)
        request = self.request_class(environ)
        response = self.get_response(request)
 
        response._handler_class = self.__class__
 
        status = "%d %s" % (response.status_code, response.reason_phrase)
        response_headers = [
            *response.items(),
            *(("Set-Cookie", c.output(header="")) for c in response.cookies.values()),
        ]
 
        start_response(status, response_headers)
        if getattr(response, "file_to_stream", None) is not None and environ.get(
            "wsgi.file_wrapper"
        ):
            # If `wsgi.file_wrapper` is used the WSGI server does not call
            # .close on the response, but on the file wrapper. Patch it to use
            # response.close instead which takes care of closing all files.
            response.file_to_stream.close = response.close
            response = environ["wsgi.file_wrapper"](
                response.file_to_stream, response.block_size
            )
        return response