Ticket #928 (defect)

The docstring for request.error_page says:

If a callable is provided, it will be called by default with keyword arguments 'status', 'message', 'traceback', and 'version', as for a string-formatting template. The callable must return a string which will be set to response.body.

The alleged requirement to return a string is different from ordinary page handlers which may return an iterable such as a list of strings.

What happens if an error page callable returns a list of strings instead of a single string? In _cperror.py, HTTPError.set_response() will set Content-Length to the number of elements in the list:

        content = self.get_error_page(self.status, traceback=tb,
                                      message=self._message)
        response.body = content
        response.headers['Content-Length'] = len(content)

A list of strings is a perfectly acceptable iterable to return to WSGI. And, in fact, the response.body provided by the error page callable is returned as-is to WSGI. Unfortunately, since HTTPError.set_response() insists on setting Content-Length based on the assumption the error page callable returns a single string, the HTTP response seen by the client will receive the entire response body but an incorrect Content-Length header.

If Content-Length were not set by HTTPError.set_response(), response.finalize() would calculate and set a correct Content-Length header.

I see no reason to require error page callables to return a single string instead of any iterable like ordinary page handlers. It is an inconsistency and a needless restriction which silently causes broken responses if not complied with.